prioritization

* I want to be able to assign higher priority to certain articles and lower the priority of others
* for backwards compatibility, assign a default priority of 500 to anything with no priority
* if I reserve e.g. even numbers for machine-assigned priority and odd numbers for user-assigned priority, I could potentially even have an automated prioritization job of some kind where I invite the LLM to try to identify elevated priority
* even/odd is overcomplicated. Just have the job assign into a fixed priority "escalation" bucket (by bucket I just mean a priority value)

No files, tests, or entry points are named. Start by locating how article priority is stored, defaulted, and used when articles are ordered; clarify whether the fixed escalation bucket is part of this change. Done means priorities can be assigned, missing priorities remain backward-compatible at 500, and ordering behavior is covered by tests.

Written by the indexing model from the issue text.