M9 S1d: split the notes.py monolith into a cohesive package

The 1574-line notes.py becomes a `notes/` package. The heavy shared logic moves
into focused modules; the route handlers + blueprint registration stay together
in __init__ so registration is trivially correct (most routes have no CI
auth-test that would otherwise catch a route silently dropping out):

- notes/_bp.py         — the Blueprint (isolated so route modules could import it
                          without a cycle; also the seam for a later route split).
- notes/serialize.py   — note (+labels/items/attachments/previews) serialization.
- notes/links.py       — [[wiki-link]] + #tag parsing and reconciliation.
- notes/recurrence.py  — recurring-reminder next-occurrence math.
- notes/helpers.py     — display-title/empty/filter/owner-fetch + filename/slug utils.
- notes/import_export.py — export markdown + Keep/native import specs + zip budget.
- notes/__init__.py    — the `/api/notes` routes + re-exports the external surface
                          (app.py imports `bp`; sync.py + tests import helpers).

Pure reorganization — no behavior change (routes/helpers moved verbatim). Callers
(app.py, sync.py, test_notes.py) are unchanged: `from thoughtsync.notes import X`
resolves via the package __init__ (rule 22 — the package replaces the module).
No import cycle (nothing in the package's dep chain imports notes; only app.py +
sync.py consume it). New test_all_note_routes_registered asserts all 29 route
endpoints are attached, so CI catches any module that fails to register. Runtime
DB behavior operator-verified on deploy (no Postgres CI lane).

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

tests/test_notes.py

@@ -31,6 +31,27 @@ def app():
     return create_app()
 
 
+def test_all_note_routes_registered(app):
+    # Guards the notes package split: every route handler must still be attached to the
+    # blueprint. A route whose module isn't imported by notes/__init__ would silently
+    # 404 at runtime, and most routes have no auth-guard test to otherwise catch it.
+    registered = {r.endpoint for r in app.url_map.iter_rules()}
+    expected = {
+        f"notes.{name}"
+        for name in (
+            "list_notes", "search_notes", "list_reminders", "complete_reminder",
+            "snooze_reminder", "export_notes", "import_notes", "list_titles",
+            "link_search", "note_backlinks", "reorder_notes", "create_note",
+            "get_note", "update_note", "list_revisions", "restore_revision",
+            "set_note_labels", "add_item", "update_item", "delete_item",
+            "reorder_items", "upload_attachment", "get_attachment",
+            "delete_attachment", "unfurl_link", "delete_preview", "trash_note",
+            "restore_note", "delete_note",
+        )
+    }
+    assert expected <= registered, f"unregistered note routes: {expected - registered}"
+
+
 def test_is_empty_note():
     assert is_empty_note(None, None)
     assert is_empty_note("", "   ")