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/004_add_redeem_code_notes.sql

@@ -0,0 +1,6 @@
+-- 为 redeem_codes 表添加备注字段
+
+ALTER TABLE redeem_codes
+ADD COLUMN IF NOT EXISTS notes TEXT DEFAULT NULL;
+
+COMMENT ON COLUMN redeem_codes.notes IS '备注说明（管理员调整时的原因说明）';