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/sync.md

@@ -401,7 +401,6 @@ no timestamp and have to decide what that meant.
 | Thing | Needs | Phase |
 |---|---|---|
 | `unread_notifications`, `unread_thread_notifications` | push rules | 8 |
-| `device_lists.changed` / `.left`, `device_one_time_keys_count` | the keyserver | 7 |
 | Remote presence | presence over federation | — |
 | Threads, and threaded receipts beyond storing `thread_id` | threads | 10 |
 | `contains_url`, `event_fields` | nothing; they are parsed and ignored | — |
@@ -410,9 +409,10 @@ no timestamp and have to decide what that meant.
 
 `device_lists` is sent as two empty lists rather than left out: a client
 reads a missing one as a malformed response, where two empty lists say
-"nothing changed". The notification counts are left out entirely instead,
-because a zero there is a claim — "you have nothing unread" — and an absent
-field is not.
+"nothing changed". How the two lists are worked out, and the key counts
+that ride along with them, are in [e2ee.md](e2ee.md). The notification
+counts are left out entirely instead, because a zero there is a claim —
+"you have nothing unread" — and an absent field is not.
 
 ## Endpoints