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/routers/strava.py

@@ -82,7 +82,7 @@ async def strava_reset(request: Request, bincio_session: Optional[str] = Cookie(
                 dt = datetime.fromisoformat(latest.replace("Z", "+00:00"))
                 last_ts = int(dt.astimezone(timezone.utc).timestamp())
                 break
-        except Exception:
+        except (OSError, json.JSONDecodeError, ValueError):
             continue
 
     if last_ts is None: