Add sequential build numbers within each project #53

Problem

Builds currently display their database IDs as build numbers. These IDs are global across projects and use a sequence allocated in blocks of 50, so users can see jumps such as #1 to #51.

Each project should have its own build counter.

Proposed behavior

• Number builds independently within each project, starting at Front page #1.
• Allocate the number when a build request is accepted into the queue.
• Keep database IDs for internal references.
• Show the project-specific number wherever a build number is displayed.
• Preserve the counter across restarts and retention cleanup. Deleted numbers must not be reused.
• Backfill existing builds in request order, using database ID order where request timestamps are unavailable.

Acceptance criteria

• Each project's builds are numbered Front page #1, Public project page #2, Admin: Image management #3, and so on.
• Builds in another project do not affect the counter.
• Concurrent requests cannot receive duplicate numbers within a project.
• Rejected requests do not consume a build number.
• Restarting the backend or deleting old builds does not reset the counter.
• Existing builds receive project-specific numbers.
• The API exposes the number and the UI displays it consistently.
• Existing build links continue to work.
• Tests cover concurrent requests, independent project counters, restart persistence and retention cleanup.