feat(backend): Add SSE endpoint for project event streaming

- Add /projects/{project_id}/events/stream SSE endpoint
- Add event_bus dependency injection
- Add project access authorization (placeholder)
- Add test event endpoint for development
- Add keepalive comments every 30 seconds
- Add reconnection support via Last-Event-ID header
- Add rate limiting (10/minute per IP)
- Mount events router in API
- Add sse-starlette dependency
- Add 19 comprehensive tests for SSE functionality

Implements #34

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

backend/app/api/dependencies/event_bus.py

@@ -0,0 +1,36 @@
+"""
+Event bus dependency for FastAPI routes.
+
+This module provides the FastAPI dependency for injecting the EventBus
+into route handlers. The event bus is a singleton that maintains
+Redis pub/sub connections for real-time event streaming.
+"""
+
+from app.services.event_bus import (
+    EventBus,
+    get_connected_event_bus as _get_connected_event_bus,
+)
+
+
+async def get_event_bus() -> EventBus:
+    """
+    FastAPI dependency that provides a connected EventBus instance.
+
+    The EventBus is a singleton that maintains Redis pub/sub connections.
+    It's lazily initialized and connected on first access, and should be
+    closed during application shutdown via close_event_bus().
+
+    Usage:
+        @router.get("/events/stream")
+        async def stream_events(
+            event_bus: EventBus = Depends(get_event_bus)
+        ):
+            ...
+
+    Returns:
+        EventBus: The global connected event bus instance
+
+    Raises:
+        EventBusConnectionError: If connection to Redis fails
+    """
+    return await _get_connected_event_bus()