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/048_add_error_passthrough_rules.sql

@@ -0,0 +1,24 @@
+-- Error Passthrough Rules table
+-- Allows administrators to configure how upstream errors are passed through to clients
+
+CREATE TABLE IF NOT EXISTS error_passthrough_rules (
+    id BIGSERIAL PRIMARY KEY,
+    name VARCHAR(100) NOT NULL,
+    enabled BOOLEAN NOT NULL DEFAULT true,
+    priority INTEGER NOT NULL DEFAULT 0,
+    error_codes JSONB DEFAULT '[]',
+    keywords JSONB DEFAULT '[]',
+    match_mode VARCHAR(10) NOT NULL DEFAULT 'any',
+    platforms JSONB DEFAULT '[]',
+    passthrough_code BOOLEAN NOT NULL DEFAULT true,
+    response_code INTEGER,
+    passthrough_body BOOLEAN NOT NULL DEFAULT true,
+    custom_message TEXT,
+    description TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Indexes for efficient queries
+CREATE INDEX IF NOT EXISTS idx_error_passthrough_rules_enabled ON error_passthrough_rules (enabled);
+CREATE INDEX IF NOT EXISTS idx_error_passthrough_rules_priority ON error_passthrough_rules (priority);