Consolidate task logic into single source of truth (DRY refactor)

This refactor eliminates duplicate task logic across the codebase by
creating a centralized task package with three layers:

- predicates/: Pure Go functions defining task state logic (IsCompleted,
  IsOverdue, IsDueSoon, IsUpcoming, IsActive, IsInProgress, EffectiveDate)
- scopes/: GORM scope functions mirroring predicates for database queries
- categorization/: Chain of Responsibility pattern for kanban column assignment

Key fixes:
- Fixed PostgreSQL DATE vs TIMESTAMP comparison bug in scopes (added
  explicit ::timestamp casts) that caused summary/kanban count mismatches
- Fixed models/task.go IsOverdue() and IsDueSoon() to use EffectiveDate
  (NextDueDate ?? DueDate) instead of only DueDate
- Removed duplicate isTaskCompleted() helpers from task_repo.go and
  task_button_types.go

Files refactored to use consolidated logic:
- task_repo.go: Uses scopes for statistics, predicates for filtering
- task_button_types.go: Uses predicates instead of inline logic
- responses/task.go: Delegates to categorization package
- dashboard_handler.go: Uses scopes for task statistics
- residence_service.go: Uses predicates for report generation
- worker/jobs/handler.go: Documented SQL with predicate references

Added comprehensive tests:
- predicates_test.go: Unit tests for all predicate functions
- scopes_test.go: Integration tests verifying scopes match predicates
- consistency_test.go: Three-layer consistency tests ensuring predicates,
  scopes, and categorization all return identical results

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/task/predicates/predicates.go

@@ -0,0 +1,189 @@
+// Package predicates provides pure predicate functions for task logic.
+// These functions are the SINGLE SOURCE OF TRUTH for all task-related business logic.
+//
+// IMPORTANT: The scopes in ../scopes/scopes.go must mirror these predicates exactly.
+// Any change to predicate logic MUST be reflected in the corresponding scope.
+// Tests verify consistency between predicates and scopes.
+package predicates
+
+import (
+	"time"
+
+	"github.com/treytartt/casera-api/internal/models"
+)
+
+// =============================================================================
+// STATE PREDICATES
+// =============================================================================
+
+// IsCompleted returns true if a task is considered "completed" per kanban rules.
+//
+// A task is completed when:
+//   - NextDueDate is nil (no future occurrence scheduled)
+//   - AND it has at least one completion record
+//
+// This applies to one-time tasks. Recurring tasks always have a NextDueDate
+// after completion, so they never enter the "completed" state permanently.
+//
+// SQL equivalent (in scopes.go ScopeCompleted):
+//
+//	next_due_date IS NULL AND EXISTS (SELECT 1 FROM task_taskcompletion tc WHERE tc.task_id = task_task.id)
+func IsCompleted(task *models.Task) bool {
+	return task.NextDueDate == nil && len(task.Completions) > 0
+}
+
+// IsActive returns true if the task is not cancelled and not archived.
+// Active tasks are eligible for display in the kanban board.
+//
+// SQL equivalent (in scopes.go ScopeActive):
+//
+//	is_cancelled = false AND is_archived = false
+func IsActive(task *models.Task) bool {
+	return !task.IsCancelled && !task.IsArchived
+}
+
+// IsCancelled returns true if the task has been cancelled.
+//
+// SQL equivalent:
+//
+//	is_cancelled = true
+func IsCancelled(task *models.Task) bool {
+	return task.IsCancelled
+}
+
+// IsArchived returns true if the task has been archived.
+//
+// SQL equivalent:
+//
+//	is_archived = true
+func IsArchived(task *models.Task) bool {
+	return task.IsArchived
+}
+
+// IsInProgress returns true if the task has status "In Progress".
+//
+// SQL equivalent (in scopes.go ScopeInProgress):
+//
+//	task_taskstatus.name = 'In Progress'
+func IsInProgress(task *models.Task) bool {
+	return task.Status != nil && task.Status.Name == "In Progress"
+}
+
+// =============================================================================
+// DATE PREDICATES
+// =============================================================================
+
+// EffectiveDate returns the date used for scheduling calculations.
+//
+// For recurring tasks that have been completed at least once, NextDueDate
+// contains the next occurrence. For new tasks or one-time tasks, we fall
+// back to DueDate.
+//
+// Returns nil if task has no due date set.
+//
+// SQL equivalent:
+//
+//	COALESCE(next_due_date, due_date)
+func EffectiveDate(task *models.Task) *time.Time {
+	if task.NextDueDate != nil {
+		return task.NextDueDate
+	}
+	return task.DueDate
+}
+
+// IsOverdue returns true if the task's effective date is in the past.
+//
+// A task is overdue when:
+//   - It has an effective date (NextDueDate or DueDate)
+//   - That date is before the given time
+//   - The task is not completed, cancelled, or archived
+//
+// SQL equivalent (in scopes.go ScopeOverdue):
+//
+//	COALESCE(next_due_date, due_date) < ?
+//	AND NOT (next_due_date IS NULL AND EXISTS completion)
+//	AND is_cancelled = false AND is_archived = false
+func IsOverdue(task *models.Task, now time.Time) bool {
+	if !IsActive(task) || IsCompleted(task) {
+		return false
+	}
+	effectiveDate := EffectiveDate(task)
+	if effectiveDate == nil {
+		return false
+	}
+	return effectiveDate.Before(now)
+}
+
+// IsDueSoon returns true if the task's effective date is within the threshold.
+//
+// A task is "due soon" when:
+//   - It has an effective date (NextDueDate or DueDate)
+//   - That date is >= now AND < (now + daysThreshold)
+//   - The task is not completed, cancelled, archived, or already overdue
+//
+// SQL equivalent (in scopes.go ScopeDueSoon):
+//
+//	COALESCE(next_due_date, due_date) >= ?
+//	AND COALESCE(next_due_date, due_date) < ?
+//	AND NOT (next_due_date IS NULL AND EXISTS completion)
+//	AND is_cancelled = false AND is_archived = false
+func IsDueSoon(task *models.Task, now time.Time, daysThreshold int) bool {
+	if !IsActive(task) || IsCompleted(task) {
+		return false
+	}
+	effectiveDate := EffectiveDate(task)
+	if effectiveDate == nil {
+		return false
+	}
+	threshold := now.AddDate(0, 0, daysThreshold)
+	// Due soon = not overdue AND before threshold
+	return !effectiveDate.Before(now) && effectiveDate.Before(threshold)
+}
+
+// IsUpcoming returns true if the task is due after the threshold or has no due date.
+//
+// A task is "upcoming" when:
+//   - It has no effective date, OR
+//   - Its effective date is >= (now + daysThreshold)
+//   - The task is not completed, cancelled, or archived
+//
+// This is the default category for tasks that don't match other criteria.
+func IsUpcoming(task *models.Task, now time.Time, daysThreshold int) bool {
+	if !IsActive(task) || IsCompleted(task) {
+		return false
+	}
+	effectiveDate := EffectiveDate(task)
+	if effectiveDate == nil {
+		return true // No due date = upcoming
+	}
+	threshold := now.AddDate(0, 0, daysThreshold)
+	return !effectiveDate.Before(threshold)
+}
+
+// =============================================================================
+// COMPLETION HELPERS
+// =============================================================================
+
+// HasCompletions returns true if the task has at least one completion record.
+func HasCompletions(task *models.Task) bool {
+	return len(task.Completions) > 0
+}
+
+// CompletionCount returns the number of completions for a task.
+func CompletionCount(task *models.Task) int {
+	return len(task.Completions)
+}
+
+// =============================================================================
+// RECURRING TASK HELPERS
+// =============================================================================
+
+// IsRecurring returns true if the task has a recurring frequency.
+func IsRecurring(task *models.Task) bool {
+	return task.Frequency != nil && task.Frequency.Days != nil && *task.Frequency.Days > 0
+}
+
+// IsOneTime returns true if the task is a one-time (non-recurring) task.
+func IsOneTime(task *models.Task) bool {
+	return !IsRecurring(task)
+}