Apply 15s timeout to subscription limit tests to stop 5s flakes

Why: active tests in the Subscription limit enforcement block lacked the
bumped timeout that the file's own comment said was needed, so the block
grant test flaked at Bun's 5s boundary.

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

Author: jahooma

web/src/app/api/v1/chat/completions/__tests__/completions.test.ts

@@ -833,7 +833,7 @@ describe('/api/v1/chat/completions POST endpoint', () => {
       expect(body.error).toBe('rate_limit_exceeded')
       expect(body.message).toContain('weekly limit reached')
       expect(body.message).toContain('Enable "Continue with credits"')
-    })
+    }, SUBSCRIPTION_TEST_TIMEOUT_MS)
 
     it('skips subscription limit check when in FREE mode even with fallback disabled', async () => {
       const weeklyLimitError: BlockGrantResult = {
@@ -880,7 +880,7 @@ describe('/api/v1/chat/completions POST endpoint', () => {
       })
 
       expect(response.status).toBe(200)
-    })
+    }, SUBSCRIPTION_TEST_TIMEOUT_MS)
 
     it('returns 429 when block exhausted and fallback disabled', async () => {
       const blockExhaustedError: BlockGrantResult = {
@@ -914,7 +914,7 @@ describe('/api/v1/chat/completions POST endpoint', () => {
       expect(body.error).toBe('rate_limit_exceeded')
       expect(body.message).toContain('5-hour session limit reached')
       expect(body.message).toContain('Enable "Continue with credits"')
-    })
+    }, SUBSCRIPTION_TEST_TIMEOUT_MS)
 
     it('continues when weekly limit reached but fallback is enabled', async () => {
       const weeklyLimitError: BlockGrantResult = {
@@ -945,7 +945,7 @@ describe('/api/v1/chat/completions POST endpoint', () => {
 
       expect(response.status).toBe(200)
       expect(mockLogger.info).toHaveBeenCalled()
-    })
+    }, SUBSCRIPTION_TEST_TIMEOUT_MS)
 
     it('continues when block grant is created successfully', async () => {
       const blockGrant: BlockGrantResult = {
@@ -977,7 +977,7 @@ describe('/api/v1/chat/completions POST endpoint', () => {
       expect(response.status).toBe(200)
       // getUserPreferences should not be called when block grant succeeds
       expect(mockGetUserPreferences).not.toHaveBeenCalled()
-    })
+    }, SUBSCRIPTION_TEST_TIMEOUT_MS)
 
     it.skip('continues when ensureSubscriberBlockGrant throws an error (fail open)', async () => {
       const mockEnsureSubscriberBlockGrant = mock(async () => {