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/integration/subscription_is_free_test.go

@@ -2,6 +2,7 @@ package integration
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
@@ -238,7 +239,7 @@ func TestIntegration_IsFreeBypassesCheckLimit(t *testing.T) {
 
 	// ========== Test 1: Normal free user hits limit ==========
 	// First property should succeed
-	err = app.SubscriptionService.CheckLimit(userID, "properties")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "properties")
 	assert.NoError(t, err, "First property should be allowed")
 
 	// Create a property to use up the limit
@@ -249,7 +250,7 @@ func TestIntegration_IsFreeBypassesCheckLimit(t *testing.T) {
 	app.DB.Create(residence)
 
 	// Second property should fail
-	err = app.SubscriptionService.CheckLimit(userID, "properties")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "properties")
 	assert.Error(t, err, "Second property should be blocked for normal free user")
 	var appErr *apperrors.AppError
 	require.ErrorAs(t, err, &appErr)
@@ -262,17 +263,17 @@ func TestIntegration_IsFreeBypassesCheckLimit(t *testing.T) {
 	require.NoError(t, err)
 
 	// ========== Test 3: IsFree user bypasses limit ==========
-	err = app.SubscriptionService.CheckLimit(userID, "properties")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "properties")
 	assert.NoError(t, err, "IsFree user should bypass property limits")
 
 	// Should also bypass other limits
-	err = app.SubscriptionService.CheckLimit(userID, "tasks")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "tasks")
 	assert.NoError(t, err, "IsFree user should bypass task limits")
 
-	err = app.SubscriptionService.CheckLimit(userID, "contractors")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "contractors")
 	assert.NoError(t, err, "IsFree user should bypass contractor limits")
 
-	err = app.SubscriptionService.CheckLimit(userID, "documents")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "documents")
 	assert.NoError(t, err, "IsFree user should bypass document limits")
 }
 
@@ -375,6 +376,6 @@ func TestIntegration_IsFreeWhenGlobalLimitationsDisabled(t *testing.T) {
 		"With IsFree and global limitations disabled, limitations_enabled should be false")
 
 	// Both cases result in the same outcome - no limitations
-	err = app.SubscriptionService.CheckLimit(userID, "properties")
+	err = app.SubscriptionService.CheckLimit(context.Background(), userID, "properties")
 	assert.NoError(t, err, "Should bypass limits when global limitations are disabled")
 }