rename to wohnungsdidi + didi logo + footer for all + seconds-only counter

- App is now called "wohnungsdidi" everywhere user-facing (page title,
  nav brand, login header, notification subjects, report filename,
  FastAPI titles, log messages)
- Brand dot replaced with an image of Didi (web/static/didi.webp),
  rendered as a round 2.25rem avatar in _layout + login
- "Programmiert für Annika ♥" footer now shows for every logged-in user,
  not only Annika
- Count-up shows only seconds ("vor 73 s") regardless of age — no
  rollover to minutes/hours
- Data continuity: DB file stays /data/lazyflat.sqlite and the Docker
  volume stays lazyflat_data so the rename doesn't strand existing data
- Session cookie renamed to wohnungsdidi_session (one-time logout on
  rollout)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/templates/login.html

@@ -3,9 +3,9 @@
 <main class="flex min-h-screen items-center justify-center p-4">
     <div class="card w-full max-w-sm p-8">
         <div class="flex items-center gap-3 mb-6">
-            <div class="brand-dot"></div>
+            <img src="/static/didi.webp" alt="" class="brand-dot">
             <div>
-                <h1 class="text-2xl font-semibold leading-tight">lazyflat</h1>
+                <h1 class="text-2xl font-semibold leading-tight">wohnungsdidi</h1>
                 <p class="text-sm text-slate-500">Anmeldung erforderlich</p>
             </div>
         </div>