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/app/api/overrides/route.ts

@@ -4,6 +4,7 @@ import type { NextRequest } from "next/server";
 import { NextResponse } from "next/server";
 
 import { withAuth } from "@/lib/auth-middleware";
+import { logger } from "@/lib/logger";
 import { createPocketBaseClient } from "@/lib/pocketbase";
 import type { OverrideType } from "@/types";
 
@@ -59,6 +60,12 @@ export const POST = withAuth(async (request: NextRequest, user) => {
     .collection("users")
     .update(user.id, { activeOverrides: newOverrides });
 
+  // Log override toggle per observability spec
+  logger.info(
+    { userId: user.id, override: overrideToAdd, enabled: true },
+    "Override toggled",
+  );
+
   return NextResponse.json({ activeOverrides: newOverrides });
 });
 
@@ -98,5 +105,11 @@ export const DELETE = withAuth(async (request: NextRequest, user) => {
     .collection("users")
     .update(user.id, { activeOverrides: newOverrides });
 
+  // Log override toggle per observability spec
+  logger.info(
+    { userId: user.id, override: overrideToRemove, enabled: false },
+    "Override toggled",
+  );
+
   return NextResponse.json({ activeOverrides: newOverrides });
 });