feat: mandatory auto-generated passwords for share links + admin password copy/regeneration + password gate rate limiting

app/migrations/applied/036_share_links_encrypted_password.sql

@@ -0,0 +1,3 @@
+-- Migration 036: add encrypted_password to share_links
+-- Required by ShareLink to store encrypted plaintext passwords for admin display.
+ALTER TABLE share_links ADD COLUMN encrypted_password TEXT;