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/admin.py

@@ -331,7 +331,7 @@ async def admin_reextract_originals(
                         total_imported += evt.get("imported", 0)
                         total_skipped += evt.get("skipped", 0)
                         total_errors += evt.get("errors", 0)
-                except Exception:
+                except json.JSONDecodeError:
                     pass
 
             await proc.wait()
@@ -390,7 +390,7 @@ async def admin_diag(
         try:
             idx = json.loads(merged_index.read_text())
             merged_activity_count = len(idx.get("activities", []))
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             merged_activity_count = -1
 
     root_activity_count: int | None = None
@@ -398,7 +398,7 @@ async def admin_diag(
         try:
             idx = json.loads(root_index.read_text())
             root_activity_count = len(idx.get("activities", []))
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             root_activity_count = -1
 
     # Peek at a few filenames in activities/ to understand the actual state
@@ -497,7 +497,7 @@ async def admin_delete_user_directory(
     from bincio.render.cli import _write_root_manifest
     try:
         _write_root_manifest(deps._get_data_dir())
-    except Exception:
+    except (OSError, json.JSONDecodeError):
         pass
     return JSONResponse({"ok": True})
 
@@ -523,7 +523,7 @@ async def admin_strava_sync_status(
                 sc = json.loads(sync_path.read_text(encoding="utf-8"))
                 last_sync = sc.get("last_sync")
                 total_imported = len(sc.get("imported_ids", []))
-            except Exception:
+            except (OSError, json.JSONDecodeError):
                 pass
 
         run_status: str | None = None
@@ -540,7 +540,7 @@ async def admin_strava_sync_status(
                 run_errors = ss.get("errors", 0)
                 run_error_message = ss.get("error_message")
                 last_run = ss.get("last_run")
-            except Exception:
+            except (OSError, json.JSONDecodeError):
                 pass
 
         users.append({