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/gassi_zeiten.py

@@ -3,7 +3,7 @@
 import json
 import logging
 from fastapi import APIRouter, Depends, HTTPException
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 from typing import Optional, List
 from database import db
 from auth import get_current_user
@@ -15,14 +15,14 @@ router = APIRouter()
 
 
 class GassiZeitCreate(BaseModel):
-    dog_id:     Optional[int]  = None
-    wochentage: List[str]       # ["mo", "mi", "fr"]
-    uhrzeit:    str             # "17:00"
-    ort_name:   Optional[str]  = None
+    dog_id:     Optional[int]   = None
+    wochentage: List[str]                                        # ["mo", "mi", "fr"]
+    uhrzeit:    str             = Field(..., max_length=20)      # "17:00"
+    ort_name:   Optional[str]   = Field(None, max_length=300)
     lat:        Optional[float] = None
     lon:        Optional[float] = None
     radius_m:   int             = 500
-    notiz:      Optional[str]  = None
+    notiz:      Optional[str]   = Field(None, max_length=2000)
 
 
 class GassiZeitUpdate(BaseModel):