[devogechukwu]

Summary

Stops /api/billing/portal from executing the billing middleware chain (billing rate limiter, REST limiter, per-developer concurrency middleware) twice by mounting the portal router inside the billing router before access log and timeout middleware. Replaces the check-then-acquire concurrency race condition with atomic synchronous slot acquisition.

Closes #[issue-number]

Changes

1. Mount Route Once:
   • src/routes/billing.ts: Mounted /portal using createBillingPortalRouter() before billingAccessLogMiddleware and createTimeoutMiddleware.
   • src/routes/index.ts: Removed the redundant separate /billing/portal mount.
   • Public paths (/api/billing/portal/*) and response shapes remain unchanged.
2. Atomic Slot Acquisition:
   • src/utils/developerSemaphore.ts: Added tryAcquire(developerId: string): (() => void) | null to synchronously verify capacity/queue and increment activeCount. Returns an idempotent release function.
   • src/middleware/perDevConcurrency.ts: Replaced getCurrentActiveSlotCounts comparison and deferred withSlot with const release = semaphore.tryAcquire(userId). Fails fast with 429 on null and guards slot release on finish and close so it releases exactly once.
3. Tests & Documentation:
   • Added unit test cases to src/middleware/perDevConcurrency.test.ts covering tryAcquire, idempotent double-release (finish followed by close), and error recovery.
   • Added src/routes/billing.portal.mount.test.ts to verify middleware chains execute exactly once on portal routes.
   • Updated docs/per-dev-concurrency.md and verified route map alignment in ARCHITECTURE.md.

Limiter Invocation Counts

Metric / Limiter	Before	After
Billing Rate Limiter invocations	2 per portal request	1 per portal request
REST Limiter invocations	2 per portal request	1 per portal request
Concurrency slot acquisition attempts	2 per portal request (causes self-429 when max=1)	1 per portal request
Billing Access Log entries	2 (1 fallthrough billing + 1 portal)	1 (portal only)
Timeout controllers attached	2	1

Closes #1503