keyserver: cache remote users' device and cross-signing keys

POST /keys/query for a remote user who shares a room with a local user is
now answered from a cache, fetched whole with GET /user/devices/{userId}
on a miss. Incoming m.device_list_update EDUs are applied when their
prev_id matches the cached stream_id; anything else marks the copy stale
and it is fetched again in the background, and a stale copy is never
served. m.signing_key_update replaces the cached cross-signing keys. A
user's copy is dropped once no local user shares a room with them, by a
keyserver listener on the roomserver's emit stage.

Migration 0009 adds keyserver_remote_users, keyserver_remote_device_keys
and keyserver_remote_cross_signing_keys for sqlite and postgres.

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

server/internal/storage/postgres/migrations/0009_remote_keys.sql

@@ -0,0 +1,48 @@
+-- The cache of other servers' users' keys.
+--
+-- A local client about to send an encrypted message asks for the keys of
+-- everyone in the room, and asks again whenever a sync says their device
+-- list moved. Answering that for a remote user by asking their server
+-- every time puts a federation round trip in front of every message, and
+-- turns a busy room into a steady stream of identical queries. So the
+-- keys of remote users who share a room with somebody here are kept, and
+-- kept current by the m.device_list_update and m.signing_key_update EDUs
+-- their server sends whenever they change.
+--
+-- These are tables of their own rather than rows in the local key tables.
+-- A local user's keys are the source of truth, and every write to them
+-- moves the device list stream and goes out over federation; a cached
+-- copy of somebody else's is neither, and keeping them apart means no
+-- code path that announces a local change can ever announce a copy.
+
+-- One row per cached user: the marker that the user's device list is held
+-- here at all, and the position in their server's device list stream the
+-- copy is at. An update names the position before it (prev_id), and is
+-- applied only when that is the position held; anything else means an
+-- update was missed, and the copy is marked stale until it is fetched
+-- again whole.
+CREATE TABLE IF NOT EXISTS keyserver_remote_users (
+    user_id   TEXT    NOT NULL PRIMARY KEY,
+    stream_id BIGINT  NOT NULL,
+    stale     BOOLEAN NOT NULL DEFAULT FALSE
+);
+
+-- The identity keys of a cached user's devices, as their server sent
+-- them, and the display name it gave each.
+CREATE TABLE IF NOT EXISTS keyserver_remote_device_keys (
+    user_id      TEXT NOT NULL,
+    device_id    TEXT NOT NULL,
+    display_name TEXT NOT NULL DEFAULT '',
+    key_json     TEXT NOT NULL,
+    PRIMARY KEY (user_id, device_id)
+);
+
+-- A cached user's master and self-signing keys. The user-signing key is
+-- never sent over federation, so there is no row for one.
+CREATE TABLE IF NOT EXISTS keyserver_remote_cross_signing_keys (
+    user_id  TEXT NOT NULL,
+    -- master or self_signing.
+    purpose  TEXT NOT NULL,
+    key_json TEXT NOT NULL,
+    PRIMARY KEY (user_id, purpose)
+);