feat(packs-lab): POST /api/packs/admin/mint-token 라우트 + 통합 테스트

MintTokenRequest/Response 스키마 추가, mint_token 라우트 구현 (HMAC 인증 + 확장자 검증 + JTI 발급), 테스트 3건 추가.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packs-lab/app/models.py

@@ -37,3 +37,17 @@ class PackFileItem(BaseModel):
     size_bytes: int
     sort_order: int
     uploaded_at: datetime
+
+
+class MintTokenRequest(BaseModel):
+    """Vercel → backend: admin upload 토큰 발급 요청."""
+    tier: PackTier
+    label: str = Field(..., max_length=200)
+    filename: str = Field(..., max_length=255)
+    size_bytes: int = Field(..., gt=0, le=5 * 1024 * 1024 * 1024)
+
+
+class MintTokenResponse(BaseModel):
+    token: str
+    expires_at: datetime
+    jti: str