add structured logging and admin diagnostics to serve

- bincio.serve logger wired into uvicorn output: rebuild steps, upload
  errors, strava-zip progress all now appear in the server log
- _trigger_rebuild: capture stdout/stderr, log errors instead of silently
  discarding; exceptions logged with traceback instead of swallowed
- upload handler: log per-file errors with traceback; include error detail
  in the SSE event sent back to the browser
- strava-zip handler: log imported/error counts on completion
- GET /api/admin/users/{handle}/diag: snapshot of a user's data dir
  (file counts, sizes, index activity counts, pending uploads)
- POST /api/admin/users/{handle}/rebuild-sync: blocking rebuild that
  returns full stdout/stderr — for debugging without SSH log access
- Admin page: Diag button per user opens a modal showing the diag JSON

bincio/serve/cli.py

@@ -76,4 +76,11 @@ def serve(data_dir: str, site_dir: Optional[str], host: str, port: int,
         console.print(f"  Users: [dim]unlimited[/dim]")
     console.print()
 
-    uvicorn.run(srv.app, host=host, port=port, log_level="info")
+    log_config = uvicorn.config.LOGGING_CONFIG.copy()
+    # Make bincio.serve logger emit at INFO through uvicorn's handler
+    log_config["loggers"]["bincio.serve"] = {
+        "handlers": ["default"],
+        "level": "INFO",
+        "propagate": False,
+    }
+    uvicorn.run(srv.app, host=host, port=port, log_level="info", log_config=log_config)