Spelunk: sign each device in through the browser instead of keeping Spelunk API keys in the vault

A Spelunk API key reaches one organisation, so a server elsewhere - oc-linux in Orange Climate - was
refused before it reached the agent. Each device now signs in to a Spelunk once, under Settings ->
Security -> Spelunk or from a Spelunk host's editor: the OAuth authorization code flow with PKCE for a
native app (RFC 8252), with Spelunk as the authorization server. The browser opens Spelunk's
/api/app-sign-in/authorize with a loopback redirect (the app's own scheme on Android), the person signs
in there however they normally do and allows "DodoSSH on <device>", and the code is redeemed at
/api/app-sign-in/token for an app session token. That token opens terminals in every organisation its
person is in, with the role they have on each server at the time; tunnel tickets send no organisation.

The sign-in stays on the device: sealed in the local cache under the LocalCacheKey (AAD resource type
18, bound to its row), never synced, wiped with the cache, and only ever sent to the Spelunk it came
from. A 401 from Spelunk forgets it; SIGN OUT ends it there and here, and so does signing out of
DodoSSH. Before every Spelunk connection DodoSSH asks its own identity provider whether its sign-in
still stands (a forced refresh, at most once a minute), and an account it refuses is signed out of
DodoSSH and of every Spelunk on the device.

A route names no key any more: Spelunk hosts are written at host schema 11, which an older build shows
read-only and asks to be updated for, and a payload that still names a key credential decodes with the
id skipped. The SpelunkKey item type is gone: SyncEntityType 14, ChangeEntityType 14 and AAD type 17
are retired, the server drops spelunk_key and its change-log rows, and the client cache drops its
cached items, outbox entries and conflicts of that type. A link still never starts a sign-in: from a
Spelunk this device is not signed in to it opens the editor, which offers to.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/DodoSSH.Contracts/SyncEntityType.cs

@@ -67,13 +67,6 @@ public enum SyncEntityType
     /// </remarks>
     ObjectStore = 13,
 
-    /// <summary>
-    /// A Spelunk API key, and the one Spelunk address it may be sent to.
-    /// </summary>
-    /// <remarks>
-    /// An item of its own rather than a credential's password, because the address is the point: a link from
-    /// Spelunk can be sent by anything on the machine, so a key may only go where the user said it goes, and
-    /// only an item that carries both can say so. See <c>SpelunkKeySecret</c>.
-    /// </remarks>
-    SpelunkKey = 14,
+    // 14 was SpelunkKey, a Spelunk API key and its address, removed while still in beta when signing in to
+    // Spelunk replaced it. Not to be reused: an old client may still send it, and must be refused as unknown.
 }