mgr: harden db_ready()/set_kv()/get_kv() against sqlite3.DatabaseError

db_ready() only caught MgrDBNotReady, so a raw sqlite3.DatabaseError
from the db property still crashed the calling module. set_kv()/get_kv()
had the same gap; give them the same @MgrModuleRecoverDB retry
protection devicehealth already uses for its own db access.

Fixes: https://tracker.ceph.com/issues/79526
Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>
Assisted-by: IBM-Bob:ClaudeSonnet