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/routes/chat.py

@@ -4,7 +4,7 @@ import os
 import uuid
 from datetime import datetime, timedelta
 from fastapi import APIRouter, Depends, HTTPException, UploadFile, File
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 from database import db
 from auth import get_current_user
 
@@ -142,7 +142,7 @@ async def get_messages(conv_id: int, offset: int = 0, limit: int = 50,
 
 
 class SendMsgModel(BaseModel):
-    text: str
+    text: str = Field(..., min_length=1, max_length=2000)
 
 
 @router.post("/conversations/{conv_id}/messages", status_code=201)
@@ -151,8 +151,6 @@ async def send_message(conv_id: int, data: SendMsgModel, user=Depends(get_curren
     text = data.text.strip()
     if not text:
         raise HTTPException(400, "Nachricht darf nicht leer sein.")
-    if len(text) > 2000:
-        raise HTTPException(400, "Nachricht zu lang (max. 2000 Zeichen).")
 
     with db() as conn:
         conv = conn.execute(