feat: note search and shareable links

Adds title search, shareable note links, and attachment downloads.

- store.search_notes filters an owner's notes by title substring
- auth.make_share_token / check_share_token back the share-link flow
- api.download_attachment streams a file from the attachment directory

tests/test_store.py

@@ -19,3 +19,9 @@ def test_notes_are_scoped_to_owner():
     s.add_note("alice", "a", "1")
     s.add_note("bob", "b", "2")
     assert len(s.notes_for("alice")) == 1
+
+
+def test_search_finds_a_note():
+    s = Store()
+    s.add_note("alice", "shopping list", "eggs")
+    assert len(s.search_notes("alice", "shopping")) == 1