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/bfl.py

@@ -0,0 +1,148 @@
+"""Async client for the BlackForestLabs image generation API."""
+
+from __future__ import annotations
+
+import asyncio
+from dataclasses import dataclass
+from typing import cast
+
+import httpx
+
+_BASE_URL = "https://api.bfl.ai"
+_DEFAULT_POLL_INTERVAL = 1.0
+_DEFAULT_TIMEOUT = 300.0
+
+
+@dataclass
+class BFLResult:
+    """Successful generation result from the BFL API."""
+
+    task_id: str
+    sample_url: str
+
+
+class BFLError(Exception):
+    """Error returned by the BFL API."""
+
+
+class BFLClient:
+    """Async client for the BlackForestLabs image generation API.
+
+    Submits generation requests and polls for results.
+    """
+
+    _api_key: str
+    _base_url: str
+    _poll_interval: float
+    _timeout: float
+
+    def __init__(
+        self,
+        api_key: str,
+        *,
+        base_url: str = _BASE_URL,
+        poll_interval: float = _DEFAULT_POLL_INTERVAL,
+        timeout: float = _DEFAULT_TIMEOUT,
+    ) -> None:
+        self._api_key = api_key
+        self._base_url = base_url
+        self._poll_interval = poll_interval
+        self._timeout = timeout
+
+    async def generate(self, model: str, inputs: dict[str, object]) -> BFLResult:
+        """Submit a generation request and poll until the result is ready.
+
+        Args:
+            model: BFL model name (e.g. ``flux-pro-1.1``, ``flux-2-pro``).
+            inputs: Model-specific parameters (prompt, dimensions, etc.).
+
+        Returns:
+            A :class:`BFLResult` containing the task ID and sample image URL.
+
+        Raises:
+            BFLError: If the API returns an error or the request times out.
+        """
+        headers = {
+            "x-key": self._api_key,
+            "Content-Type": "application/json",
+            "Accept": "application/json",
+        }
+
+        async with httpx.AsyncClient(
+            base_url=self._base_url, headers=headers
+        ) as client:
+            task_id, polling_url = await self._submit(client, model, inputs)
+            sample_url = await self._poll(client, task_id, polling_url)
+
+        return BFLResult(task_id=task_id, sample_url=sample_url)
+
+    async def _submit(
+        self,
+        client: httpx.AsyncClient,
+        model: str,
+        inputs: dict[str, object],
+    ) -> tuple[str, str]:
+        """POST the generation request and return (task_id, polling_url)."""
+        response = await client.post(f"/v1/{model}", json=inputs)
+
+        if response.status_code == 422:  # noqa: PLR2004
+            body = cast(dict[str, object], response.json())
+            detail = body.get("detail", body)
+            msg = f"BFL validation error for model '{model}': {detail}"
+            raise BFLError(msg)
+
+        if response.status_code != 200:  # noqa: PLR2004
+            msg = f"BFL API returned status {response.status_code}: {response.text}"
+            raise BFLError(msg)
+
+        body = cast(dict[str, object], response.json())
+        task_id = body.get("id")
+        polling_url = body.get("polling_url")
+
+        if not isinstance(task_id, str) or not isinstance(polling_url, str):
+            msg = f"BFL API response missing 'id' or 'polling_url': {body}"
+            raise BFLError(msg)
+
+        return task_id, polling_url
+
+    async def _poll(
+        self,
+        client: httpx.AsyncClient,
+        task_id: str,
+        polling_url: str,
+    ) -> str:
+        """Poll the task until ready and return the sample image URL."""
+        elapsed = 0.0
+
+        while elapsed < self._timeout:
+            response = await client.get(polling_url)
+
+            if response.status_code != 200:  # noqa: PLR2004
+                msg = f"BFL polling returned status {response.status_code}: {response.text}"
+                raise BFLError(msg)
+
+            body = cast(dict[str, object], response.json())
+            status = body.get("status")
+
+            if status == "Ready":
+                result = body.get("result")
+                if not isinstance(result, dict):
+                    msg = f"BFL task {task_id} ready but no result dict: {body}"
+                    raise BFLError(msg)
+                result_dict = cast(dict[str, object], result)
+                sample = result_dict.get("sample")
+                if not isinstance(sample, str):
+                    msg = f"BFL task {task_id} ready but no sample URL: {result}"
+                    raise BFLError(msg)
+                return sample
+
+            if status in ("Error", "Failed"):
+                error_msg = body.get("result", body)
+                msg = f"BFL task {task_id} failed: {error_msg}"
+                raise BFLError(msg)
+
+            await asyncio.sleep(self._poll_interval)
+            elapsed += self._poll_interval
+
+        msg = f"BFL task {task_id} timed out after {self._timeout}s"
+        raise BFLError(msg)