After the PgCryptoStore migration in v0.8.0, the verify_with_recovery_key
call that previously ran after share_keys() was dropped. On any rotation
that uploads fresh device keys (fresh crypto.db, server had stale keys
from a prior install, etc.), the new device keys carry no valid self-
signing signature because the bot has no access to the self-signing
private key.
Peers like Element then refuse to share Megolm sessions with the
rotated device, so the bot silently stops decrypting incoming messages.
This restores the recovery-key bootstrap: on startup, if
MATRIX_RECOVERY_KEY is set, import the cross-signing private keys from
SSSS and sign_own_device(), producing a valid signature server-side.
Idempotent and gated on MATRIX_RECOVERY_KEY — no behavior change for
users who don't configure a recovery key.
Verified end-to-end by deleting crypto.db and restarting: the bot
rotates device identity keys, re-uploads, self-signs via recovery key,
and decrypts+replies to fresh messages from a paired Element client.
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