Onboarding: template backlink, bulk-create endpoint, climate-region scoring

Clients that send users through a multi-task onboarding step no longer
loop N POST /api/tasks/ calls and no longer create "orphan" tasks with
no reference to the TaskTemplate they came from.

Task model
- New task_template_id column + GORM FK (migration 000016)
- CreateTaskRequest.template_id, TaskResponse.template_id
- task_service.CreateTask persists the backlink

Bulk endpoint
- POST /api/tasks/bulk/ — 1-50 tasks in a single transaction,
  returns every created row + TotalSummary. Single residence access
  check, per-entry residence_id is overridden with batch value
- task_handler.BulkCreateTasks + task_service.BulkCreateTasks using
  db.Transaction; task_repo.CreateTx + FindByIDTx helpers

Climate-region scoring
- templateConditions gains ClimateRegionID; suggestion_service scores
  residence.PostalCode -> ZipToState -> GetClimateRegionIDByState against
  the template's conditions JSON (no penalty on mismatch / unknown ZIP)
- regionMatchBonus 0.35, totalProfileFields 14 -> 15
- Standalone GET /api/tasks/templates/by-region/ removed; legacy
  task_tasktemplate_regions many-to-many dropped (migration 000017).
  Region affinity now lives entirely in the template's conditions JSON

Tests
- +11 cases across task_service_test, task_handler_test, suggestion_
  service_test: template_id persistence, bulk rollback + cap + auth,
  region match / mismatch / no-ZIP / unknown-ZIP / stacks-with-others

Docs
- docs/openapi.yaml: /tasks/bulk/ + BulkCreateTasks schemas, template_id
  on TaskResponse + CreateTaskRequest, /templates/by-region/ removed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

migrations/000017_drop_task_template_regions_join.down.sql

@@ -0,0 +1,12 @@
+-- Recreates the legacy task_tasktemplate_regions join table. Data is not
+-- restored — if a rollback needs the prior associations they have to be
+-- reseeded from the task template conditions JSON.
+
+CREATE TABLE IF NOT EXISTS task_tasktemplate_regions (
+    task_template_id  BIGINT NOT NULL,
+    climate_region_id BIGINT NOT NULL,
+    PRIMARY KEY (task_template_id, climate_region_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_task_tasktemplate_regions_region
+    ON task_tasktemplate_regions (climate_region_id);