feat(auth): add WebAuthn passkey support with migrations, routes, and tests

backend/migrations/202512010140_add_webauthn_challenges/down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS webauthn_challenges;

backend/migrations/202512010140_add_webauthn_challenges/up.sql

@@ -0,0 +1,13 @@
+CREATE TABLE webauthn_challenges (
+    id UUID PRIMARY KEY,
+    user_id UUID REFERENCES users(id) ON DELETE CASCADE,
+    purpose TEXT NOT NULL,
+    challenge BYTEA NOT NULL,
+    state BYTEA NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    expires_at TIMESTAMPTZ NOT NULL,
+    CONSTRAINT webauthn_challenges_purpose_check CHECK (purpose IN ('registration', 'authentication'))
+);
+
+CREATE INDEX webauthn_challenges_user_id_idx ON webauthn_challenges (user_id);
+CREATE INDEX webauthn_challenges_expires_at_idx ON webauthn_challenges (expires_at);