Direct Question.post relations

[hlbmtc]

Added a direct Question.post FK to replace the old QuestionPost database view. This simplifies queries and improves performance.

Changes:

• New Question.post FK with related_name="questions"
• Removed QuestionPost model entirely
• Post.save() auto-syncs question FKs on creation
• Replaced all related_questions__question__X lookups with questions__X
• Replaced all related_posts lookups with posts
• Optimized batch jobs (job_compute_movement, hotness, indexes) to use direct questions relation instead of prefetch_questions() when we don't need group/conditional structure

Queries are simpler, no more view indirection, and batch jobs skip loading unnecessary models.

closes #4031

[lsabor]

This is a good change in my opinion. Code is fine and relatively simple.

Running the site locally works with no glitches. I checked the most important things such as browsing questions and notebooks, forecasting, resolving, leaderboard updating. I didn't check the more nuanced features like notifications and indexes, though I suspect they still work as expected.

Not that it's critical, but rolling back the migrations also worked properly.

I didn't benchmark anything to actually evaluate efficiencies, but nothing worked slower to the point where I could notice during normal use of the site.

[elisescu]

LGTM