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/034_ops_upstream_error_events.sql

@@ -0,0 +1,9 @@
+-- Add upstream error events list (JSONB) to ops_error_logs for per-request correlation.
+--
+-- This is intentionally idempotent.
+
+ALTER TABLE ops_error_logs
+    ADD COLUMN IF NOT EXISTS upstream_errors JSONB;
+
+COMMENT ON COLUMN ops_error_logs.upstream_errors IS
+    'Sanitized upstream error events list (JSON array), correlated per gateway request (request_id/client_request_id); used for per-request upstream debugging.';