feat(api): add Phase 3 observability - expose validation warnings and metadata to frontend

Backend changes:
- Create ResponseMetadata and TokenMetrics models for API responses
- Modify call_gemini() and call_gemini_with_function_tools() to return (response, log_id) tuple
- Add _build_response_metadata() helper to extract metadata from AICallLog
- Update routines API (/suggest, /suggest-batch) to populate validation_warnings, auto_fixes_applied, and metadata
- Update products API (/suggest) to populate observability fields
- Update skincare API to handle new return signature

Frontend changes:
- Add TypeScript types: TokenMetrics, ResponseMetadata
- Update RoutineSuggestion, BatchSuggestion, ShoppingSuggestionResponse with observability fields

Next: Create UI components to display warnings, reasoning chains, and token metrics

frontend/src/lib/types.ts

@@ -241,10 +241,29 @@ export interface RoutineSuggestionSummary {
   confidence: number;
 }
 
+// Phase 3: Observability metadata types
+export interface TokenMetrics {
+  prompt_tokens: number;
+  completion_tokens: number;
+  thoughts_tokens?: number;
+  total_tokens: number;
+}
+
+export interface ResponseMetadata {
+  model_used: string;
+  duration_ms: number;
+  reasoning_chain?: string;
+  token_metrics?: TokenMetrics;
+}
+
 export interface RoutineSuggestion {
   steps: SuggestedStep[];
   reasoning: string;
   summary?: RoutineSuggestionSummary;
+  // Phase 3: Observability fields
+  validation_warnings?: string[];
+  auto_fixes_applied?: string[];
+  metadata?: ResponseMetadata;
 }
 
 export interface DayPlan {
@@ -257,6 +276,10 @@ export interface DayPlan {
 export interface BatchSuggestion {
   days: DayPlan[];
   overall_reasoning: string;
+  // Phase 3: Observability fields
+  validation_warnings?: string[];
+  auto_fixes_applied?: string[];
+  metadata?: ResponseMetadata;
 }
 
 // ─── Shopping suggestion types ───────────────────────────────────────────────
@@ -274,6 +297,10 @@ export interface ProductSuggestion {
 export interface ShoppingSuggestionResponse {
   suggestions: ProductSuggestion[];
   reasoning: string;
+  // Phase 3: Observability fields
+  validation_warnings?: string[];
+  auto_fixes_applied?: string[];
+  metadata?: ResponseMetadata;
 }
 
 // ─── Health types ────────────────────────────────────────────────────────────