perf(subscription-status): cache + parallelize + invalidate on mutations

GET /api/subscription/status/ was the slowest endpoint in the API at
p50≈1750ms / p95≈2425ms — about 12× the floor for our cluster→Neon
geography. Jaeger traces showed seven sequential SQL queries each
costing roughly one transatlantic RTT (~110ms), with the actual queries
running in 0.073ms at the database. Pure network serialization, not slow
SQL.

Three changes, in order of leverage:

1. Cache the assembled SubscriptionStatusResponse per-user in Redis with
   a 5-minute TTL. Hot path collapses to a single Redis GET (~5ms) on
   warm reads; the TTL is a safety net against missed invalidations.

2. Parallelize the three independent COUNT queries in getUserUsage
   (task_task / task_contractor / task_document) via golang.org/x/sync
   errgroup. Three RTTs collapse to one. Also dropped the redundant
   residence_residence COUNT — len(residenceIDs) from FindResidenceIDsByOwner
   is the same number, no need to re-query.

3. Wire explicit invalidation into every mutation that could change a
   user's response — residence/task/contractor/document CRUD,
   residence membership changes (JoinWithCode, RemoveUser, DeleteResidence),
   and every subscription tier flip across the IAP/Stripe/webhook surface.
   Residence-scoped invalidations fan out to every user with access via a
   new ResidenceRepository.FindUserIDsByResidence helper, so members of a
   shared residence don't see stale `usage` numbers when another member
   adds a task.

Net effect: warm path goes from ~1350ms to ~5ms (Redis hit). Cold path
goes from ~1350ms to ~250-450ms (5 sequential queries → 2 phases:
residence IDs lookup, then parallel task/contractor/document counts).

Also fixed a pre-existing CheckLimit signature drift in
internal/integration/subscription_is_free_test.go that was blocking the
package build.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/repositories/residence_repo.go

@@ -157,6 +157,24 @@ func (r *ResidenceRepository) GetResidenceUsers(residenceID uint) ([]models.User
 	return users, nil
 }
 
+// FindUserIDsByResidence returns the IDs of every user with access to the
+// residence (owner + members from residence_residence_users). Lighter than
+// GetResidenceUsers — selects only the ID column, no full user records.
+// Used to fan out subscription_status cache invalidation when shared data
+// (tasks/contractors/documents) changes for a residence.
+func (r *ResidenceRepository) FindUserIDsByResidence(residenceID uint) ([]uint, error) {
+	var ids []uint
+	err := r.db.Raw(`
+		SELECT owner_id FROM residence_residence WHERE id = ? AND is_active = true
+		UNION
+		SELECT user_id  FROM residence_residence_users WHERE residence_id = ?
+	`, residenceID, residenceID).Scan(&ids).Error
+	if err != nil {
+		return nil, err
+	}
+	return ids, nil
+}
+
 // HasAccess checks if a user has access to a residence
 func (r *ResidenceRepository) HasAccess(residenceID, userID uint) (bool, error) {
 	var count int64