Sicherheit + Tests + A11y, SW by-v1118

PYDANTIC max_length (38 Routen, ~400 Field-Constraints):
Schützt vor DoS durch Riesen-Payloads (10MB Thread-Titel etc.).
Pragmatische Limits:
- Titel/Name: 200 · Beschreibung/Body: 10000 · Notiz: 5000
- Email: 254 (RFC 5321) · URL: 500 · Slug/Kategorie: 100
- Hund-Name/Rasse: 80 · Hund-Bio: 2000

Top-betroffen: forum.py, diary.py, health.py, dogs.py, expenses.py,
notes.py, auth.py, profile.py. Manuelle len()-Checks in profile,
chat, ki entfernt (jetzt durch Field abgedeckt).

PYTEST COVERAGE (+19 Tests, 37 grün + 1 xfail):
- test_security.py: require_owner (Places GET/PATCH/DELETE mit
  Fremduser → 403), JWT-Blacklist (Logout invalidiert Token),
  Login-Lockout (5 Fehlversuche → 429 + Retry-After Header)
- test_race.py: Invoice-Counter (20 parallele Threads, alle unique),
  Founder-Number (atomare Vergabe, voll bei 100)
- test_validation.py: Forum-Titel 30k Zeichen → 422, Diary-Text
  50k → 422 (verifiziert Pydantic max_length-Sweep)

A11Y (Tap-Targets ≥44×44 + Dark-Mode-Kontrast):
- #header-user-btn 36→44px, .header-back 40→44, .header-menu-btn 40→44
- dog-profile Wrapped-Slider Prev/Next 40→44
- forum-Lightbox Close 40→44
- --c-text-muted Light: #B0A090 (2.37:1 FAIL) → #7F6B58 (4.74:1 PASS)
- --c-text-muted Dark:  #806A58 (3.58:1 FAIL) → #A08878 (5.46:1 PASS)
- Branding-Farben unangetastet

backend/static/js/pages/forum.js

@@ -1421,7 +1421,7 @@ function _fmtDate(iso) {
     const lb = document.createElement('div');
     lb.style.cssText = 'position:fixed;inset:0;background:rgba(0,0,0,.92);z-index:9999;display:flex;align-items:center;justify-content:center;cursor:zoom-out';
     lb.innerHTML = `<img src="${UI.escape(src)}" style="max-width:100%;max-height:100%;object-fit:contain;touch-action:pinch-zoom">
-      <button style="position:absolute;top:16px;right:16px;background:rgba(255,255,255,.2);border:none;border-radius:50%;width:40px;height:40px;color:#fff;font-size:22px;cursor:pointer;display:flex;align-items:center;justify-content:center">✕</button>`;
+      <button style="position:absolute;top:16px;right:16px;background:rgba(255,255,255,.2);border:none;border-radius:50%;width:44px;height:44px;color:#fff;font-size:22px;cursor:pointer;display:flex;align-items:center;justify-content:center">✕</button>`;
     lb.addEventListener('click', () => lb.remove());
     document.body.appendChild(lb);
   }