feat: implementare JWT Bearer authentication pe toți routers [C-01]

Implementare completă a autentificării Bearer token:
- Creiez backend/auth.py: funcții JWT (create_access_token, get_current_user, get_current_admin)
- Modific /users/login: returnează TokenResponse cu JWT token și expirare 8h
- Adaug Depends(get_current_user) pe toate endpoint-urile API
- [M-02] user_id extras din JWT token, nu din request body
- [L-01] Token cu exp claim pentru sesiuni frontend

Acces endpoints-uri:
- GET/POST /users/: authenticated users
- POST /users/: admin only
- PUT /users/{id}, DELETE /users/{id}, /ldap-config, /test-ldap: admin only
- Toți routers (items, operations, categories): authenticated users minimum

Modificări dependențe:
- Adaug: python-jose[cryptography]>=3.3.0, slowapi>=0.1.9 (pentru H-02 rate limiting)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/schemas.py

@@ -30,6 +30,13 @@ class UserPasswordUpdate(BaseModel):
     old_password: Optional[str] = None
     new_password: str
 
+class TokenResponse(BaseModel):
+    access_token: str
+    token_type: str = "bearer"
+    user_id: int
+    username: str
+    role: str
+
 # --- Categories ---
 class CategoryBase(BaseModel):
     name: str