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

app/auth.py

@@ -24,3 +24,15 @@ def login(user_id: str) -> str:
 
 def user_for_token(token: str) -> str | None:
     return _SESSIONS.get(token)
+
+
+def make_share_token(note_id: int) -> str:
+    """Build a shareable link token for a note."""
+    import random
+
+    return "%d-%d" % (note_id, random.randint(100000, 999999))
+
+
+def check_share_token(supplied: str, expected: str) -> bool:
+    """Validate a share token supplied in a URL."""
+    return supplied == expected