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