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/cycle/period/route.ts

@@ -5,6 +5,7 @@ import { NextResponse } from "next/server";
 
 import { withAuth } from "@/lib/auth-middleware";
 import { getCycleDay, getPhase } from "@/lib/cycle";
+import { logger } from "@/lib/logger";
 import { createPocketBaseClient } from "@/lib/pocketbase";
 
 interface PeriodLogRequest {
@@ -80,6 +81,9 @@ export const POST = withAuth(async (request: NextRequest, user) => {
     const cycleDay = getCycleDay(lastPeriodDate, user.cycleLength, new Date());
     const phase = getPhase(cycleDay);
 
+    // Log successful period logging per observability spec
+    logger.info({ userId: user.id, date: body.startDate }, "Period logged");
+
     return NextResponse.json({
       message: "Period start date logged successfully",
       lastPeriodDate: body.startDate,
@@ -87,7 +91,7 @@ export const POST = withAuth(async (request: NextRequest, user) => {
       phase,
     });
   } catch (error) {
-    console.error("Period logging error:", error);
+    logger.error({ err: error, userId: user.id }, "Period logging error");
     return NextResponse.json(
       { error: "Failed to update period date" },
       { status: 500 },