add JobModel: persistent scheduler job store (#2360, phase 1)

[ebuzerdrmz44]

Description

First PR of the scheduler refactor (#2360), which I'm splitting into small stacked PRs (full breakdown is on the issue). This one just adds JobModel; nothing reads or writes it yet.

No migration needed. connection.py builds tables with create_tables(), which is safe by default, so the table shows up on both fresh and existing DBs.

The idea: JobModel tracks a job's lifecycle and intent (scheduled, running, done, skipped, interrupted). When a job actually runs it links to the matching EventLogModel row through event_log, so I'm not duplicating start/end/returncode across two tables. EventLogModel stays the record of what executed, and JobModel adds the intent plus the things that never produce a log line, like a job that got skipped.

A few schema questions before I build on it. The rest of the plan doesn't depend on these, but this table's shape does:

1. Store scheduled jobs as rows, or keep deriving the next run on reload like today? The issue wants pending jobs to survive restarts, but storing them risks a second source of truth against the derivation in set_timer_for_profile. I lean toward persisting skipped and interrupted jobs while leaving next-run derivation as is.
2. FK or plain strings for profile/repo? EventLogModel uses strings, so its rows survive a profile being deleted. Same for the jobs history, or is a FK fine here?
3. For crash recovery, is a startup sweep that marks leftover running jobs as interrupted enough, or do you want more than that?

@m3nu If you'd rather this not merge as a bare model, I can fold in the first consumer (skip-reason recording) before then.