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/Cargo.toml

@@ -9,6 +9,11 @@ description = "Axum backend: CalDAV proxy, sessions, preferences, ICS feeds."
 runway-core = { workspace = true, features = ["ical", "recurrence"] }
 runway-caldav = { workspace = true }
 axum = { workspace = true }
+axum-extra = { workspace = true }
+base64 = { workspace = true }
+chacha20poly1305 = { workspace = true }
+time = "0.3"
+rand = { workspace = true }
 chrono = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
@@ -25,6 +30,7 @@ uuid = { workspace = true }
 [dev-dependencies]
 reqwest = { workspace = true }
 pretty_assertions = { workspace = true }
+tower = { workspace = true, features = ["util"] }
 
 [lints]
 workspace = true