Phase 7: E2EE support

Implements PLAN.md Phase 7: the server half of end-to-end encryption.
Devices upload identity, one-time and fallback keys; other devices query
and claim them, across servers too, and every one-time key is handed out
exactly once. Cross-signing, signature uploads, device list tracking and
server-side key backup complete the set.

internal/keyserver is the component. Keys are stored in canonical JSON
so that re-uploading the same key is a byte comparison, and signatures
uploaded later are kept apart from the key they sign and merged in on
read, so a key's own JSON stays what its owner uploaded. Device keys must
be for the caller's own device; cross-signing keys must be the
uploader's, carry the right usage and, for self- and user-signing keys,
be signed by the master key. Signatures are verified against the key as
stored. Replacing a master key asks for the password; a first upload or
an upload of the keys already stored does not, as the spec allows. A
signature of another user's master key is shown only to the user who
made it and never federated.

Every write that changes what /keys/query answers moves the device list
stream in the same transaction, one position per device, and wakes the
syncs of everyone sharing a room. The stream is the sixth field of a sync
token; older tokens still parse. The syncapi works out device_lists
changed and left from the stream and from membership moves in the
window, skipping invites and knocks, and reports
device_one_time_keys_count and device_unused_fallback_key_types, the
latter as [] when empty. GET /keys/changes uses the same logic.

Over federation: /user/keys/query, /user/keys/claim and
/user/devices/{userId}, and m.device_list_update and m.signing_key_update
both ways. Remote keys are not cached; an update about a remote user
sharing a room moves them in the stream so local clients re-query.
Device display names are not sent to other servers. With federation off,
remote users are reported in failures at once.

Deleting a device deletes its keys, through a new userapi
DeviceObserver. Key backup keeps the better copy of each session by the
spec's rule, never reuses a deleted version's number, and answers writes
to a stale version with M_WRONG_ROOM_KEYS_VERSION.

Storage is migration 0008 for sqlite and postgres, with a PocketBase
stub. The Complement target sets gain the key, device list, fallback,
cross-signing and backup tests.

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

docs/federation.md

@@ -288,7 +288,8 @@ way rather than processed twice.
 | `m.typing` | a local user's typing, to the room's servers | into the typing tracker, if the user belongs to the sending server |
 | `m.receipt` | a local user's public read receipts — never private ones | into the receipts stream, likewise |
 | `m.direct_to_device` | to-device messages for remote users, one EDU per server | delivered to local devices |
-| `m.presence`, `m.device_list_update` | — | ignored: presence is off by default, device lists are Phase 7 |
+| `m.device_list_update`, `m.signing_key_update` | a local user's key changes, to every server sharing a room with them | the remote user is marked changed for local syncs; see [e2ee.md](e2ee.md) |
+| `m.presence` | — | ignored: presence is off by default |
 
 Nothing in an EDU is signed, so the one check there is — the user must
 belong to the server that sent it — is what keeps a server from making
@@ -363,8 +364,8 @@ and `GET /profile/{user}` for a remote user.
   target arrives later is not applied when the target arrives.
 - **Room versions 1 and 2** — event IDs carried in the event — are not
   supported, so the oldest rooms on the network cannot be joined.
-- **Presence and device list EDUs** are ignored on the way in and not sent
-  (presence is off by default; device lists are Phase 7).
+- **Presence EDUs** are ignored on the way in and not sent (presence is
+  off by default).
 - **Third-party invites** across servers (`exchange_third_party_invite`)
   and **knocking** over federation (`make_knock`, `send_knock`) are not
   implemented.