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/me.py

@@ -151,7 +151,7 @@ async def me_delete_account(
     from bincio.render.cli import _write_root_manifest
     try:
         _write_root_manifest(deps._get_data_dir())
-    except Exception:
+    except (OSError, json.JSONDecodeError):
         pass
 
     resp = JSONResponse({"ok": True})
@@ -214,7 +214,7 @@ async def me_get_strava_credentials(bincio_session: str | None = Cookie(default=
             if cid and csec:
                 has_user_creds = True
                 client_id_hint = cid
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             pass
     return JSONResponse({
         "has_user_creds": has_user_creds,
@@ -242,7 +242,7 @@ async def me_set_strava_credentials(
             try:
                 existing = json.loads(creds_path.read_text(encoding="utf-8"))
                 csec = str(existing.get("client_secret", "")).strip()
-            except Exception:
+            except (OSError, json.JSONDecodeError):
                 pass
     if not csec:
         raise HTTPException(400, "client_secret is required (no existing secret to preserve)")
@@ -255,7 +255,7 @@ async def me_set_strava_credentials(
             old_cid = str(json.loads(creds_path.read_text(encoding="utf-8")).get("client_id", "")).strip()
             if old_cid and old_cid != cid:
                 token_path.unlink(missing_ok=True)
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             pass
 
     creds_path.write_text(