Implement structured logging for API routes (P3.7)

Replace console.error with pino structured logger across API routes
and add key event logging per observability spec:
- Auth failure (warn): reason
- Period logged (info): userId, date
- Override toggled (info): userId, override, enabled
- Decision calculated (info): userId, decision, reason
- Error events (error): err object with stack trace

Files updated:
- auth-middleware.ts: Added structured logging for auth failures
- cycle/period/route.ts: Added Period logged event + error logging
- calendar/[userId]/[token].ics/route.ts: Replaced console.error
- overrides/route.ts: Added Override toggled events
- today/route.ts: Added Decision calculated event

Tests: 720 passing (added 3 new structured logging tests)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/lib/auth-middleware.ts

@@ -7,6 +7,7 @@ import { NextResponse } from "next/server";
 
 import type { User } from "@/types";
 
+import { logger } from "./logger";
 import {
   createPocketBaseClient,
   getCurrentUser,
@@ -54,19 +55,21 @@ export function withAuth<T = unknown>(
 
       // Check if the user is authenticated
       if (!isAuthenticated(pb)) {
+        logger.warn({ reason: "not_authenticated" }, "Auth failure");
         return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
       }
 
       // Get the current user
       const user = getCurrentUser(pb);
       if (!user) {
+        logger.warn({ reason: "user_not_found" }, "Auth failure");
         return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
       }
 
       // Call the original handler with the user context
       return await handler(request, user, context);
     } catch (error) {
-      console.error("Auth middleware error:", error);
+      logger.error({ err: error }, "Auth middleware error");
       return NextResponse.json(
         { error: "Internal server error" },
         { status: 500 },