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

backend/migrations/202512010130_add_user_passkeys/down.sql

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+DROP TABLE IF EXISTS user_passkeys;

backend/migrations/202512010130_add_user_passkeys/up.sql

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+CREATE TABLE user_passkeys (
+    id UUID PRIMARY KEY,
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    credential_id BYTEA NOT NULL UNIQUE,
+    public_key BYTEA NOT NULL,
+    credential JSONB NOT NULL,
+    sign_count BIGINT NOT NULL,
+    transports TEXT[] NOT NULL DEFAULT '{}',
+    aaguid UUID,
+    nickname TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_used_at TIMESTAMPTZ,
+    revoked_at TIMESTAMPTZ,
+    revoked_by UUID,
+    revoked_reason TEXT
+);
+
+CREATE INDEX user_passkeys_user_id_idx ON user_passkeys (user_id);