Add authentication

Proving who somebody is and starting a session for them are separate
operations. login_with_caldav does the first by asking the CalDAV server
whether the credentials work; begin_session does the second and knows
nothing about how the question was answered. OIDC arrives as a second way
to reach begin_session, not as a second scheme threaded through everything
-- which is what v1 had, with a JWT for most of the app and a separate
SQLite session_token used only by the preferences API.

The token lives in an HttpOnly cookie and nowhere else. v1 kept a JWT and
the CalDAV password in localStorage, readable by any script on the origin,
and re-sent the password in a header on every request. Here the password
never leaves the server: it is encrypted with XChaCha20-Poly1305 and
caldav_for is the only path back, handing out a client rather than a
credential.

Failed decryption is an error, not a subtly wrong password -- the AEAD tag
is checked, so a tampered row surfaces here instead of as a mysterious
CalDAV rejection later. A wrong password and an unreachable server stay
distinct, because telling somebody their password is wrong when the server
is down sends them to reset one that was fine.

Errors carry a stable code alongside their message, so a client can branch
on them. Internal ones say nothing about the inside of the server; the
detail goes to the log.

Tests go through router(), the same function main calls -- v1's suite
rebuilt the route table and tested a copy until it stopped compiling.
Skipping is now loud: a skipped test reports "ok", so run.sh sets
RUNWAY_REQUIRE_CALDAV=1 and not running becomes a failure.

crates/runway-server/src/routes/mod.rs

@@ -0,0 +1,12 @@
+//! HTTP handlers.
+
+pub mod auth;
+
+use axum::Json;
+use serde_json::{Value, json};
+
+/// Liveness. Says nothing about the database on purpose: a health check that
+/// touches storage turns a slow query into an outage.
+pub async fn health() -> Json<Value> {
+    json!({ "status": "ok", "service": "runway", "version": env!("CARGO_PKG_VERSION") }).into()
+}