refactor: route TPM reservation and settlement through the derived value

Five call sites (QuotaGate entry, RetryMW/EmbeddingClient success and
transient-failure settlement) now read effective_est_tokens() instead of
est_tokens. Success paths gain an unavailable branch that keeps delta at
zero once usage frames may be missing; it has no producer yet, so
behaviour is unchanged while the tpm>0 => est_tokens>0 gate still holds.

src/polygateway/middleware/ratelimit.py

@@ -23,7 +23,7 @@ class QuotaGate:
 
     async def try_acquire(self, source: SourceConfig) -> Permit | None:
         try:
-            return await self._limiter.try_acquire(source.name, source.est_tokens)
+            return await self._limiter.try_acquire(source.name, source.effective_est_tokens())
         except GovernanceBackendError:
             raise
         except Exception as exc: