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/activities.py

@@ -192,7 +192,7 @@ async def delete_activity(
             idx = json.loads(index_path.read_text(encoding="utf-8"))
             idx["activities"] = [a for a in idx.get("activities", []) if a.get("id") != activity_id]
             index_path.write_text(json.dumps(idx, indent=2, ensure_ascii=False))
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             pass  # corrupt index — merge_all will clean up on next run
 
     # Remove from dedup cache so the file can be re-uploaded if needed
@@ -205,7 +205,7 @@ async def delete_activity(
                     a for a in cache["activities"] if a.get("id") != activity_id
                 ]
                 cache_path.write_text(json.dumps(cache, indent=2, ensure_ascii=False))
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             pass  # corrupt cache — leave it; next extract will rebuild
 
     # Full merge needed: activity removed from index
@@ -289,13 +289,13 @@ async def get_athlete(bincio_session: str | None = Cookie(default=None)) -> JSON
     # Layer edits/athlete.yaml on top
     edits_path = dd / "edits" / "athlete.yaml"
     if edits_path.exists():
+        import yaml
         try:
-            import yaml
             edits = yaml.safe_load(edits_path.read_text(encoding="utf-8")) or {}
             for k in ("max_hr", "ftp_w", "hr_zones", "power_zones", "seasons", "gear"):
                 if k in edits:
                     data[k] = edits[k]
-        except Exception:
+        except (OSError, yaml.YAMLError):
             pass
     return JSONResponse(data)