Feat: Serve images directly from Cloudflare R2 CDN

API responses now include file_url and thumbnail_url fields. When
S3_PUBLIC_BASE_URL is configured, these point to the CDN domain;
when unset, they fall back to the existing API proxy paths so local
dev requires no additional setup. UI updated to use response URL
fields directly instead of constructing proxy URLs client-side.

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

api/app/config.py

@@ -14,6 +14,7 @@ class Settings(BaseSettings):
     s3_secret_access_key: str
     s3_region: str = "us-east-1"
     api_base_url: str = "http://localhost:8000"
+    s3_public_base_url: str | None = None
     max_upload_bytes: int = 52_428_800  # 50 MiB
     jwt_secret_key: str
     jwt_expiry_seconds: int = 86400