Refactor step 4: narrow broad except Exception catches

Replaced 28 bare `except Exception` catches across 8 files with specific
exception types reflecting the actual failure modes:

- JSON file reads → (OSError, json.JSONDecodeError)
- datetime parsing → ValueError
- base64 decoding → ValueError
- YAML parsing → (OSError, yaml.YAMLError); import moved above try
- GeoJSON coord extraction → (TypeError, IndexError, AttributeError)
- Startup temp-file cleanup → OSError
- Single JSON line parsing (SSE batch) → json.JSONDecodeError

Kept broad catches only where intentional:
- Background thread top-level guards (tasks.py, admin.py) with log.exception
- SSE stream generator tops (strava.py, garmin.py, uploads.py)
- Per-item batch loops that must not abort the whole operation
- Explicitly non-fatal post-upload merge steps with log.warning

bincio/serve/server.py

@@ -39,7 +39,7 @@ async def _on_startup() -> None:
     for p in _glob.glob(str(data_dir / "*" / "tmp*.zip")):
         try:
             Path(p).unlink()
-        except Exception:
+        except OSError:
             pass
     if deps.webroot is not None:
         threading.Thread(target=tasks._site_rebuild_worker, daemon=True, name="site-rebuild").start()