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/ideas.py

@@ -37,7 +37,7 @@ async def list_ideas(
     for path in sorted(_ideas_dir(dd).glob("*.json")):
         try:
             idea = json.loads(path.read_text(encoding="utf-8"))
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             continue
         votes = idea.get("votes", [])
         idea["vote_count"] = len(votes)
@@ -161,7 +161,7 @@ async def delete_idea(
         raise HTTPException(404, "Not found")
     try:
         idea = json.loads(path.read_text(encoding="utf-8"))
-    except Exception:
+    except (OSError, json.JSONDecodeError):
         raise HTTPException(500, "Could not read idea")
     if not user.is_admin and idea.get("author") != user.handle:
         raise HTTPException(403, "Forbidden")
@@ -221,7 +221,7 @@ async def submit_feedback(
     if log_file.exists():
         try:
             existing = json.loads(log_file.read_text())
-        except Exception:
+        except (OSError, json.JSONDecodeError):
             existing = []
     existing.append(entry)
     log_file.write_text(json.dumps(existing, indent=2))