feat: migrate one-time and signup tokens to an actor (#1611)

Co-authored-by: Elias Schneider <login@eliasschneider.com>

backend/resources/migrations/sqlite/20260723000000_actor_tokens.up.sql

@@ -0,0 +1,35 @@
+PRAGMA foreign_keys=OFF;
+BEGIN;
+
+-- One-time access tokens are now stored in the actor state store, so the table is no longer needed.
+DROP TABLE IF EXISTS one_time_access_tokens;
+
+-- Freeze the signup tokens.
+-- Encode every signup token (with its user group IDs) as a single JSON array and store it in the "kv" table under the "signup_tokens_migrated" key, so the singleton signup token actor can seed its state from it on first startup.
+-- The "HAVING count(*) > 0" clause ensures nothing is written to the "kv" table when there are no signup tokens.
+-- Timestamps are stored as Unix seconds, matching how DateTime values are persisted on SQLite.
+INSERT INTO kv ("key", "value")
+SELECT 'signup_tokens_migrated', json_group_array(
+    json_object(
+        'id', st.id,
+        'token', st.token,
+        'expiresAt', st.expires_at,
+        'usageLimit', st.usage_limit,
+        'usageCount', st.usage_count,
+        'createdAt', st.created_at,
+        'userGroupIds', json((
+            SELECT COALESCE(json_group_array(stug.user_group_id), json_array())
+            FROM signup_tokens_user_groups stug
+            WHERE stug.signup_token_id = st.id
+        ))
+    )
+)
+FROM signup_tokens st
+HAVING count(*) > 0;
+
+-- Drop the now-frozen signup token tables.
+DROP TABLE signup_tokens_user_groups;
+DROP TABLE signup_tokens;
+
+COMMIT;
+PRAGMA foreign_keys=ON;