Prototype paused admin participant AI support

Enable an existing AI difficulty for a human seat through admin controls without granting private access or advancing paused play. Preserve custody, credentials, takeback and separate paced resume. Bind atomic writes and outgoing-grant revocation to live authority, room/pause versions and active seat access with durable exact receipts.

Validation: types/lint, 5408 offline tests, build and 52 HTTP tests passed on source fingerprint 2157de0f4bb0b74d9760d52102601c52344cf674dee8ccd1d559f0a246a1d4ff; 22 authenticated admin HTTP groups, independent SQL/privacy/UI review, desktop/mobile and refresh checks. All 2098 baseline rooms and 4455 seats preserved. Migration 0015 is additive. Positive production admin QA awaits authorized access; full admin/rules scope remains unfinished.

README.md

@@ -53,7 +53,9 @@ use the Directory filter to find them for restoration.
 readable tables and history, then resumes the same saved position.
 [Archive or unarchive](docs/ADMIN_ROOM_ARCHIVE.md) organizes closed rooms while
 retaining their tables and history; unarchiving leaves play closed.
-Permanent deletion, participant support and backup tools are still being built.
+[Participant AI](docs/ADMIN_PARTICIPANT_AI.md) supports an absent human in a paused
+game while preserving their seat and takeback control. Permanent deletion,
+further participant support and backup tools are still being built.
 
 ## Saved seats and reconnecting