M6 1902a: richer facet query + saved-filters storage (backend)

GET /api/notes gains combinable, AND-ed facets alongside the existing
filter/date/sort: multiple ?label= (notes with ALL), ?color, ?kind,
?has_reminder, ?has_attachment, and ?q (full-text over title+body, ranked)
— so the facet bar's text box searches, not just filters. All optional;
invalid color/kind → 400.

saved_filters table (migration 0021) + /api/saved-filters CRUD (list /
create / rename+repoint / delete, owner-scoped). `params` is a JSON facet
dict mirroring the query surface; clean_params() whitelists facet keys so a
saved view can't accumulate junk.

Tests (DB-free): _truthy, clean_params key-whitelisting, saved-filters
auth-guards. UI (facet bar + saved-views sidebar) lands next.

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

src/thoughtsync/models/all.py

@@ -13,6 +13,7 @@ from . import (  # noqa: F401
     note_link,
     note_link_preview,
     note_revision,
+    saved_filter,
     settings,
     share,
     user,