perf(task): offload completion notification fan-out to Asynq worker

POST /api/task-completions/ was spending ~1.5-1.75s synchronously on
APNs push + SMTP email + B2 image fetches inside sendTaskCompletedNotification.
Per-user loop made it scale linearly with residence membership; one image
attached + one residence user is the 1.75s baseline observed in the live
honeydue-eli5-overview Grafana panel.

Replace the inline call (and the fire-and-forget goroutine in QuickComplete,
which violated the project's "no goroutines in handlers" rule) with an
Asynq job:

  - new task type notification:task_completed (worker/scheduler.go)
  - new payload {task_id, completion_id} — IDs only, worker re-reads
    canonical state from Postgres so concurrent edits between enqueue
    and dequeue are reflected
  - new HandleTaskCompletedNotification on jobs.Handler delegates to
    TaskService.SendTaskCompletedNotificationByID
  - new dispatchTaskCompletedNotification in task_service.go picks
    between enqueue (preferred) and inline (fallback) when Redis is
    unreachable or the enqueuer isn't wired (tests / local dev)

Other changes required to wire it up:

  - widen worker.NewTaskClient signature to accept asynq.RedisClientOpt
    so the file-mounted Redis password (audit HIGH-1) can be supplied;
    no prior callers, no breakage
  - extend worker.Enqueuer interface with EnqueueTaskCompletedNotification
  - add TaskEnqueuer field to router.Dependencies; wire from cmd/api/main.go
    with the standard typed-nil interface guard
  - wire a worker-side TaskService in cmd/worker/main.go so the handler
    can use the shared SendTaskCompletedNotificationByID implementation
    (storage service shared with the existing upload-cleanup wiring)

Expected impact on POST /api/task-completions/ p50:
  ~1.75s -> ~120-170ms (DB + tx + Asynq enqueue only)

Notifications still deliver; they just go via the worker instead of in
the request path. MaxRetry=3; "row not found" returns nil so a deleted
task/completion doesn't churn the retry loop.

All 31 test packages pass. No DB migrations.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/worker/enqueuer.go

@@ -2,12 +2,16 @@ package worker
 
 import "encoding/json"
 
-// Enqueuer defines the interface for enqueuing background email tasks.
+// Enqueuer defines the interface for enqueuing background email + notification
+// tasks from the api request path. Implementations are expected to be cheap to
+// call and non-blocking (Asynq's client batches over a persistent Redis
+// connection).
 type Enqueuer interface {
 	EnqueueWelcomeEmail(to, firstName, code string) error
 	EnqueueVerificationEmail(to, firstName, code string) error
 	EnqueuePasswordResetEmail(to, firstName, code, resetToken string) error
 	EnqueuePasswordChangedEmail(to, firstName string) error
+	EnqueueTaskCompletedNotification(taskID, completionID uint) error
 }
 
 // Verify TaskClient satisfies the interface at compile time.
@@ -42,3 +46,12 @@ func BuildPasswordResetEmailPayload(to, firstName, code, resetToken string) ([]b
 func BuildPasswordChangedEmailPayload(to, firstName string) ([]byte, error) {
 	return json.Marshal(EmailPayload{To: to, FirstName: firstName})
 }
+
+// BuildTaskCompletedNotificationPayload marshals a TaskCompletedNotificationPayload
+// to JSON bytes for the Asynq queue.
+func BuildTaskCompletedNotificationPayload(taskID, completionID uint) ([]byte, error) {
+	return json.Marshal(TaskCompletedNotificationPayload{
+		TaskID:       taskID,
+		CompletionID: completionID,
+	})
+}