fix: resolve P0/P1 code quality issues

P0 fixes:
- ModelError.Is(): use exact matching instead of substring contains()
- shouldClearStickySession: add context param for cancellation/tracing

P1 fixes:
- TODO stubs: return 501 Not Implemented errors
- validateInstanceSignature: deduplicate to shared validateCodeSignature()
- Error messages: standardize to English only
- http.go: remove pseudo if-else with duplicate branches

backend/migrations/074_add_usage_log_endpoints.sql

@@ -0,0 +1,5 @@
+-- Add endpoint tracking fields to usage_logs.
+-- inbound_endpoint: client-facing API route (e.g. /v1/chat/completions, /v1/messages, /v1/responses)
+-- upstream_endpoint: normalized upstream route (e.g. /v1/responses)
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS inbound_endpoint VARCHAR(128);
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS upstream_endpoint VARCHAR(128);