fix: potenitally fix e2ee from keybackups

TODO.md

@@ -40,7 +40,7 @@ Tested against real Dendrite DB dump (bocken.org):
 |---|---|---|
 | userapi_accounts | users | password_hash, created_ts (ms->s), account_type->is_guest/admin |
 | userapi_profiles | profiles | user_id=localpart, full_user_id=@user:server |
-| userapi_devices | devices | direct map |
+| userapi_devices | devices + access_tokens | direct map; access_token preserved so clients don't re-login |
 | roomserver_rooms | rooms | room_id, room_version; creator from m.room.create events |
 | roomserver_events + event_json | events + event_json | denormalize NIDs, topological_ordering=depth |
 | syncapi_current_room_state | current_state_events | direct map |
@@ -134,6 +134,24 @@ Tested against real Dendrite DB dump (bocken.org):
 - 2262 rejected events in Dendrite skipped during migration
 - 5548 orphan event edges (referencing federated events we don't have) - normal
 - Synapse background updates recalculate some stats after startup - normal
+- E2EE: three things must be migrated together for encrypted history to survive —
+  (1) `userapi_devices.access_token` -> `access_tokens` so clients don't re-login
+      (re-login usually wipes the local Megolm store and always changes device_id,
+      breaking Olm continuity),
+  (2) `syncapi_send_to_device` -> `device_inbox` so undelivered m.room.encrypted
+      Olm messages (Megolm key shares to offline devices) reach the recipient,
+  (3) `device_lists_stream` seeded from local devices so clients re-verify
+      e2e_device_keys_json on first sync (otherwise: stale cache mismatches).
+  v1 of the migration only moved the Synapse-native E2EE tables; that left
+  1480 pending to-device messages stranded and all clients forced to re-login,
+  which is the root cause of "partial key loss" reported against v1.
+- E2EE: `keyserver_fallback_keys` -> `e2e_fallback_keys_json` added so new Olm
+  sessions still succeed after OTKs are exhausted.
+- E2EE: `e2e_cross_signing_keys.stream_id` now drawn from the matching
+  sequence (`e2e_cross_signing_keys_sequence`) to avoid UNIQUE(stream_id)
+  collisions on subsequent Synapse writes.
+- Re-running Phase 8 duplicates `e2e_cross_signing_signatures` rows (no unique
+  constraint on the Synapse side). TRUNCATE before re-running, or run once.
 
 ## Usage