Add passkeys (WebAuthn): register from the profile, log in without email

New library dependency: lbuchs/webauthn (^2.2, MIT, zero transitive deps
beyond PHP+OpenSSL+Mbstring, both already required). 'none' attestation --
this only confirms "the same device that registered", not hardware
provenance, the standard trust model for a public site's own passkey login.

Backend
- migrations/010: `passkeys` (one row per registered credential: owner,
  credential_id, public_key, sign_count, label) and `webauthn_challenges`
  (short-lived, single-use, bridging each ceremony's "options" and "verify"
  calls -- user_id set for a registration, null for a login since who's
  signing in isn't known until the credential comes back).
- Config: WEBAUTHN_RP_ID (defaults to APP_URL's host) and WEBAUTHN_RP_NAME.
- PasskeyRepository, WebAuthnChallengeRepository, PasskeyController:
  GET/POST /api/passkeys, POST /api/passkeys/options, DELETE
  /api/passkeys/{id} (all auth), plus the public POST /api/auth/passkey/
  options and /verify for login. Registration always asks for a
  discoverable, user-verified credential -- what makes login usernameless:
  the browser offers whatever passkeys it has for the site, no email first.
- SessionPayload now also exposes `has_passkey` on every user object
  (PasskeyRepository::countForUser() > 0), reused by both the profile page
  and the dismissible notice.
- PasskeyTest: auth guards, options response shape, challenge single-use/
  expiry/purpose/cross-user rules, malformed-input handling, list/remove
  CRUD (seeded rows) -- everything short of a real signature, which isn't
  practical from PHPUnit. 73 tests pass.

Frontend
- lib/webauthn.ts: base64url <-> ArrayBuffer conversion and the two
  ceremonies (registerPasskey, loginWithPasskey), matching the API's wire
  format exactly.
- ProfileView: a Passkeys section -- list with Remove buttons, an "Add a
  passkey" form (label pre-filled from a UA guess).
- LoginView: a "Log in with a passkey" button above the email form, shown
  only when the browser supports WebAuthn.
- PasskeyNotice.vue: dismissible banner across the top of the page
  (`user.has_passkey === false`); dismissal is a week-long localStorage
  timestamp.

Verified against the rebuilt container using a Chrome DevTools Protocol
*virtual authenticator* (real ECDSA signing, no human interaction) end to
end: notice shown -> register a passkey -> notice gone (same page and after
navigating) -> log out -> "Log in with a passkey" with no email typed ->
correct account, notice still gone -> remove the passkey -> notice back ->
dismiss -> stays hidden for ~7 days across pages. Along the way, caught and
fixed a real bug: AuthenticatorData::getCredentialId() returns a raw binary
string, not a ByteBuffer like most of this library's other binary fields --
bin2hex() it directly rather than calling ->getHex().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

migrations/010_add_passkeys.sql

@@ -0,0 +1,30 @@
+-- Passkeys (WebAuthn discoverable credentials): an alternative to the email
+-- magic link. A user may register several (one per device/authenticator).
+CREATE TABLE IF NOT EXISTS passkeys (
+    id            INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id       INTEGER NOT NULL REFERENCES users (id) ON DELETE CASCADE,
+    credential_id TEXT NOT NULL UNIQUE,
+    public_key    TEXT NOT NULL,
+    sign_count    INTEGER NOT NULL DEFAULT 0,
+    label         TEXT NOT NULL DEFAULT '',
+    created_at    TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    last_used_at  TEXT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_passkeys_user ON passkeys (user_id);
+
+-- Short-lived, single-use WebAuthn challenges bridging the "options" and
+-- "verify" calls of both the registration and login ceremonies. user_id is
+-- set for a registration (tied to the signed-in caller) and NULL for a login
+-- attempt, since who's logging in isn't known until the credential comes back.
+CREATE TABLE IF NOT EXISTS webauthn_challenges (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    user_id     INTEGER NULL REFERENCES users (id) ON DELETE CASCADE,
+    purpose     TEXT NOT NULL CHECK (purpose IN ('register', 'login')),
+    challenge   TEXT NOT NULL,
+    expires_at  TEXT NOT NULL,
+    consumed_at TEXT NULL,
+    created_at  TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_webauthn_challenges_expiry ON webauthn_challenges (expires_at);