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/calendar/[userId]/[token].ics/route.ts

@@ -3,6 +3,7 @@
 import { type NextRequest, NextResponse } from "next/server";
 
 import { generateIcsFeed } from "@/lib/ics";
+import { logger } from "@/lib/logger";
 import { createPocketBaseClient } from "@/lib/pocketbase";
 
 interface RouteParams {
@@ -61,8 +62,7 @@ export async function GET(_request: NextRequest, { params }: RouteParams) {
       return NextResponse.json({ error: "User not found" }, { status: 404 });
     }
 
-    // Re-throw unexpected errors
-    console.error("Calendar feed error:", error);
+    logger.error({ err: error, userId }, "Calendar feed error");
     return NextResponse.json(
       { error: "Internal server error" },
       { status: 500 },