Store key_prefix for readable key display instead of masked hash

The last-4 of the SHA-256 hash was meaningless for identification.
Now storing the first 12 chars of the plaintext key as key_prefix,
displayed as 'sk-aBcDeFgH••••••••' — consistent with what the user
sees at creation time and how GitHub/OpenAI handle it.

backend/crud.py

@@ -40,6 +40,7 @@ def create_api_key(
     db_key = APIKey(
         name=name,
         key=_hash_api_key(raw_key),
+        key_prefix=raw_key[:12],
         expires_at=expires_at,
         daily_tokens=daily_tokens,
         monthly_tokens=monthly_tokens,

backend/models.py

@@ -10,6 +10,7 @@ class APIKey(Base):
     id = Column(Integer, primary_key=True, index=True)
     name = Column(String)
     key = Column(String, unique=True, index=True)
+    key_prefix = Column(String)
     is_active = Column(Boolean, default=True)
     created_at = Column(DateTime(timezone=True), default=_now)
     expires_at = Column(DateTime(timezone=True), nullable=True)

backend/schemas.py

@@ -14,6 +14,7 @@ class APIKey(BaseModel):
     id: int
     name: str
     key: str
+    key_prefix: Optional[str] = None
     is_active: bool
     created_at: datetime
     expires_at: Optional[datetime] = None

frontend/src/main.jsx

@@ -3,7 +3,7 @@ import ReactDOM from 'react-dom/client';
 import axios from 'axios';
 import './styles.css';
 
-const maskKey = (key) => `••••••••${key.slice(-4)}`;
+const displayKey = (prefix) => prefix ? `${prefix}••••••••` : '••••••••••••';
 
 function authHeaders(token) {
   return { Authorization: `Bearer ${token}` };
@@ -298,7 +298,7 @@ function App() {
               <tr key={key.id}>
                 <td>{key.id}</td>
                 <td>{key.name}</td>
-                <td>{maskKey(key.key)}</td>
+                <td>{displayKey(key.key_prefix)}</td>
                 <td>{key.is_active ? 'Aktiv' : 'Inaktiv'}</td>
                 <td>{key.expires_at ? new Date(key.expires_at).toLocaleDateString('de-DE', { timeZone: 'Europe/Berlin' }) : '∞'}</td>
                 <td>{key.daily_tokens ?? '∞'}</td>