WIP: refactor worker DB data preloading

[krokicki]

This is a work in progress!

Gets rid of the backwards RPC calls from worker to the main process. The refactoring can probably go further to minimize what is sent to the workers, I need to take a longer look at that.

Summary

• replace worker reverse DB RPC with parent-prepared request payloads
• preload file share paths and minimal job metadata before dispatching worker actions
• keep workers free of DB credentials while restoring a simple one-request / one-response IPC flow

Validation

• pixi run test-backend tests/test_worker.py -q
• pixi run test-backend tests/test_apps.py tests/test_apps_endpoints.py -q
• pixi run test-backend tests/test_endpoints.py -q -k 'not root_endpoint and not spa_routing'

Note: full tests/test_endpoints.py in the temporary worktree only failed the two SPA/static asset checks because the worktree lacked built frontend assets.