Implement V2 registration support

migrations/sqlite/2026-09-20-120000_user_crypto_v2/up.sql

@@ -0,0 +1,13 @@
+ALTER TABLE users ADD COLUMN signed_public_key TEXT;
+ALTER TABLE users ADD COLUMN security_state TEXT;
+ALTER TABLE users ADD COLUMN security_version INTEGER;
+
+CREATE TABLE user_signature_key_pairs (
+    uuid                TEXT     NOT NULL PRIMARY KEY,
+    user_uuid           TEXT     NOT NULL UNIQUE REFERENCES users (uuid) ON DELETE CASCADE,
+    signature_algorithm INTEGER  NOT NULL, -- 0 = ed25519, 1 = mldsa44
+    signing_key         TEXT     NOT NULL,
+    verifying_key       TEXT     NOT NULL,
+    created_at          DATETIME NOT NULL,
+    updated_at          DATETIME NOT NULL
+);