feat(infra): add new infrastructure layer for storage and streaming

Add app/infra/ package with:
- storage/ - repository adapters for runs, run_events, thread_meta
- run_events/ - JSONL-based event store with factory
- stream_bridge/ - memory and redis adapters for SSE streaming

This layer provides the persistence abstractions used by the gateway
services, replacing the old deerflow/persistence modules.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/infra/stream_bridge/factory.py

@@ -0,0 +1,50 @@
+"""App-owned stream bridge factory."""
+
+from __future__ import annotations
+
+import logging
+from collections.abc import AsyncIterator
+from contextlib import AbstractAsyncContextManager, asynccontextmanager
+
+from deerflow.config.stream_bridge_config import get_stream_bridge_config
+from deerflow.runtime.stream_bridge import StreamBridge
+
+from .adapters import MemoryStreamBridge, RedisStreamBridge
+
+logger = logging.getLogger(__name__)
+
+
+def build_stream_bridge(config=None) -> AbstractAsyncContextManager[StreamBridge]:
+    """Build the configured app-owned stream bridge."""
+    return _build_stream_bridge_impl(config)
+
+
+@asynccontextmanager
+async def _build_stream_bridge_impl(config=None) -> AsyncIterator[StreamBridge]:
+    if config is None:
+        config = get_stream_bridge_config()
+
+    if config is None or config.type == "memory":
+        maxsize = config.queue_maxsize if config is not None else 256
+        bridge = MemoryStreamBridge(queue_maxsize=maxsize)
+        await bridge.start()
+        logger.info("Stream bridge initialised: memory (queue_maxsize=%d)", maxsize)
+        try:
+            yield bridge
+        finally:
+            await bridge.close()
+        return
+
+    if config.type == "redis":
+        if not config.redis_url:
+            raise ValueError("Redis stream bridge requires redis_url")
+        bridge = RedisStreamBridge(redis_url=config.redis_url)
+        await bridge.start()
+        logger.info("Stream bridge initialised: redis (%s)", config.redis_url)
+        try:
+            yield bridge
+        finally:
+            await bridge.close()
+        return
+
+    raise ValueError(f"Unknown stream bridge type: {config.type!r}")