refactor: split api dependencies by domain

app/api/presentation/__init__.py

@@ -0,0 +1 @@
+"""API presentation helpers isolated from domain dependency assembly."""

app/api/presentation/sse.py

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+"""SSE wire mapping and response lifecycle helpers."""
+
+import json
+from collections.abc import AsyncIterable, Iterable
+from typing import Any
+
+from fastapi.responses import StreamingResponse
+
+
+SSE_HEADERS = {
+    "Cache-Control": "no-cache, no-transform",
+    "Connection": "keep-alive",
+    "X-Accel-Buffering": "no",
+}
+
+
+def encode_data_event(payload: dict[str, Any]) -> str:
+    """Encode an OpenAI-style unnamed SSE data event."""
+    return f"data: {json.dumps(payload, ensure_ascii=False)}\n\n"
+
+
+def encode_named_event(event: str, payload: dict[str, Any]) -> str:
+    """Encode a named SSE event without applying a response envelope."""
+    return (
+        f"event: {event}\n"
+        f"data: {json.dumps(payload, ensure_ascii=False)}\n\n"
+    )
+
+
+def build_sse_response(
+    content: AsyncIterable[str] | Iterable[str],
+    *,
+    headers: dict[str, str] | None = None,
+) -> StreamingResponse:
+    """Build a non-buffered SSE response and merge protocol-specific headers."""
+    return StreamingResponse(
+        content,
+        media_type="text/event-stream",
+        headers={**SSE_HEADERS, **(headers or {})},
+    )
+
+
+def build_sse_error_response(payload: str) -> StreamingResponse:
+    """Build a one-event SSE error response using the common transport policy."""
+    return build_sse_response(iter([payload]))