S1: unify error responses + id parsing (responses.py) across notes.py

M9 section S1, commit 2. Add src/thoughtsync/responses.py — the app's single
JSON-error shape and the two guards that pair with it:
  - json_error(message, status): the one ({"error": ...}, code) builder
  - not_found(): the standard 404, by far the most common note-route error
  - parse_uuid(raw): parse a path/body id, None on malformed → pair with not_found()

notes.py adopts them everywhere: ~25 hand-built `jsonify({"error":"not found"}),404`
collapse to not_found(); ~20 other error returns to json_error(...); ~13 repeated
`try: uuid.UUID(x) except: ...` blocks to parse_uuid(). Behavior-preserving — same
bodies and status codes, one definition. jsonify stays for the success responses.

Other blueprints (auth, labels, saved_filters, sync, settings_api) adopt the same
helpers in their own M9 sections.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/responses.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+import uuid
+
+from quart import jsonify
+
+# The app's single JSON error shape + the two guards that pair with it, so every
+# blueprint returns errors and parses path ids the same way instead of hand-building
+# `jsonify({"error": ...}), code` and try/except uuid blocks at ~35 call sites.
+
+
+def json_error(message: str, status: int):
+    """A JSON error body + status: ({"error": message}, status)."""
+    return jsonify({"error": message}), status
+
+
+def not_found():
+    """404 with the standard body — by far the most common error in the note routes."""
+    return json_error("not found", 404)
+
+
+def parse_uuid(raw: object) -> uuid.UUID | None:
+    """Parse a path/body UUID, returning None on anything malformed. Pair with
+    not_found() for the ubiquitous 'bad id in the URL → 404' guard."""
+    try:
+        return uuid.UUID(str(raw))
+    except (ValueError, TypeError):
+        return None