feat(files): share-link tokens are kept only hashed and encrypted, and every link change is audited

A /watch link's token is the credential, and it sat in the database as written. Migration 0107
replaces the column with the token's SHA-256 (the lookup) and a Fernet-encrypted copy (so the owner
can copy the same link again); every existing address keeps working, and the downgrade decrypts
them back. Without a copy this key can read, a link still opens but its address cannot be shown:
"Copy link" then takes the newest link that has one or makes a new one, and the Share dialog says
so in that row instead of a broken URL. Making, changing and revoking a link are audited, never
with its token or password.

backend/app/audit.py

@@ -73,6 +73,11 @@ class AuditAction:
     # What a token then did is the file rows above, their summary naming it.
     API_TOKEN_CREATE = "api_token_create"
     API_TOKEN_REVOKE = "api_token_revoke"
+    # A public `/watch` link to a file (`routes.downloads`): made, its settings changed, revoked.
+    # Never its token or password — only whether it has one.
+    DOWNLOAD_LINK_CREATE = "download_link_create"
+    DOWNLOAD_LINK_UPDATE = "download_link_update"
+    DOWNLOAD_LINK_REVOKE = "download_link_revoke"
 
 
 def record(