refactor: replace blackforest package with custom async BFL client

Implement bulkgen/providers/bfl.py with a fully async httpx-based client
that supports all current and future BFL models (including flux-2-*).
Remove the blackforest dependency and simplify the image provider by
eliminating the asyncio.to_thread wrapper.

bulkgen/providers/image.py

@@ -2,24 +2,15 @@
 
 from __future__ import annotations
 
-import asyncio
 import base64
 from pathlib import Path
 from typing import override
 
 import httpx
-from blackforest import BFLClient  # pyright: ignore[reportMissingTypeStubs]
-from blackforest.types.general.client_config import (  # pyright: ignore[reportMissingTypeStubs]
-    ClientConfig,
-)
-from blackforest.types.responses.responses import (  # pyright: ignore[reportMissingTypeStubs]
-    SyncResponse,
-)
 
 from bulkgen.config import TargetConfig
 from bulkgen.providers import Provider
-
-_BFL_SYNC_CONFIG = ClientConfig(sync=True, timeout=300)
+from bulkgen.providers.bfl import BFLClient
 
 
 def _encode_image_b64(path: Path) -> str:
@@ -61,24 +52,10 @@ class ImageProvider(Provider):
             ctrl_path = project_dir / control_name
             inputs["control_image"] = _encode_image_b64(ctrl_path)
 
-        result = await asyncio.to_thread(
-            self._client.generate, resolved_model, inputs, _BFL_SYNC_CONFIG
-        )
-
-        if not isinstance(result, SyncResponse):
-            msg = (
-                f"BFL API returned unexpected response type for target '{target_name}'"
-            )
-            raise RuntimeError(msg)
-
-        result_dict: dict[str, str] = result.result  # pyright: ignore[reportUnknownMemberType, reportUnknownVariableType]
-        image_url = result_dict.get("sample")
-        if not image_url:
-            msg = f"BFL API did not return an image URL for target '{target_name}'"
-            raise RuntimeError(msg)
+        result = await self._client.generate(resolved_model, inputs)
 
         async with httpx.AsyncClient() as http:
-            response = await http.get(image_url)
+            response = await http.get(result.sample_url)
             _ = response.raise_for_status()
 
         _ = output_path.write_bytes(response.content)