refactor: Move background tasks to server lifespan and deprecate SSE transport

- Move scanner/processor tasks from FastMCP session lifespan to Starlette
  server lifespan (correct architecture: background tasks run once at
  server level, not per-session)
- Change default CLI transport from SSE to streamable-http
- Remove SSE transport option from CLI (SSE is deprecated)
- Remove SSE client session factory from test fixtures
- Add tracing instrumentation to BM25 hybrid search operations for
  better observability

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

nextcloud_mcp_server/cli.py

@@ -29,9 +29,9 @@ from .app import get_app
 @click.option(
     "--transport",
     "-t",
-    default="sse",
+    default="streamable-http",
     show_default=True,
-    type=click.Choice(["sse", "streamable-http", "http"]),
+    type=click.Choice(["streamable-http", "http"]),
     help="MCP transport protocol",
 )
 @click.option(