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

.github/audit-exceptions.yml

@@ -0,0 +1,16 @@
+version: 1
+exceptions:
+  - package: xlsx
+    advisory: "GHSA-4r6h-8v6p-xvw6"
+    severity: high
+    reason: "Admin export only; switched to dynamic import to reduce exposure (CVE-2023-30533)"
+    mitigation: "Load only on export; restrict export permissions and data scope"
+    expires_on: "2026-04-05"
+    owner: "security@your-domain"
+  - package: xlsx
+    advisory: "GHSA-5pgg-2g8v-p4x9"
+    severity: high
+    reason: "Admin export only; switched to dynamic import to reduce exposure (CVE-2024-22363)"
+    mitigation: "Load only on export; restrict export permissions and data scope"
+    expires_on: "2026-04-05"
+    owner: "security@your-domain"