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/services/contractor_service.go

@@ -146,6 +146,15 @@ func (s *ContractorService) CreateContractor(ctx context.Context, req *requests.
 		return nil, apperrors.Internal(reloadErr)
 	}
 
+	// contractors_count for every user with access to this residence just
+	// changed. Contractor without a residence is rare (created via global
+	// add) and only the creator counts it — drop only their cache then.
+	if req.ResidenceID != nil {
+		invalidateSubStatusForResidence(ctx, s.cache, s.residenceRepo, *req.ResidenceID)
+	} else if s.cache != nil {
+		_ = s.cache.InvalidateSubscriptionStatusForUsers(ctx, userID)
+	}
+
 	resp := responses.NewContractorResponse(contractor)
 	return &resp, nil
 }
@@ -258,6 +267,12 @@ func (s *ContractorService) DeleteContractor(ctx context.Context, contractorID,
 		return apperrors.Internal(err)
 	}
 
+	if contractor.ResidenceID != nil {
+		invalidateSubStatusForResidence(ctx, s.cache, s.residenceRepo, *contractor.ResidenceID)
+	} else if s.cache != nil {
+		_ = s.cache.InvalidateSubscriptionStatusForUsers(ctx, userID)
+	}
+
 	return nil
 }