Follows #2928

Main changes:

• Fixes ProjectModel.default_gateway being selected for every api endpoint leading to the most expensive query (if counted by sum(duration)).
• Avoids loading unnecessary attributes in locking/preflight selects in background processing (load only .id now for locking).
• Rewrites process_fleets to multiple process fleets in one session (as it can be done quickly), reducing total number of background transactions.
• Adds DB indexes for status columns (should help with background select since filtering by active status would typically be beneficial).