fix(#891): Replace TimeStampedModel inheritance with explicit fields and indexes

[vjpixel]

Summary

Alternative implementation of #891 addressing @pablodiegoss's review on #921.

Instead of layering Meta.indexes on top of TimeStampedModel inheritance (the #921 approach), this PR removes the inheritance entirely and declares created / modified manually on each affected model. This frees the project from a forever-dependency on TimeStampedModel's shape while still adding the indexes the issue asks for.

Changes

• Removes TimeStampedModel from 8 models: PostImage, Clipping, Post (blog), Sound, Marker, Object, Artwork, Exhibit (core).
• Adds created = DateTimeField(auto_now_add=True) and modified = DateTimeField(auto_now=True) to each.
• Adds Meta.get_latest_by = "modified" (preserves migration history).
• Adds descending Meta.indexes on -created and -modified for all 8 models.
• Adds composite (exhibit_type, -created) index on Exhibit covering users/views.py:109,112 and similar queries in core/views/views.py.
• Keeps django-extensions installed (pyproject.toml, INSTALLED_APPS) so historical migrations (blog/0008, core/0013, 0016, 0027) still replay on fresh local environments.

Migrations

• src/blog/migrations/0010_* — 7 AlterField + 6 AddIndex.
• src/core/migrations/0028_* — 20 AlterField (10 main models + 10 pghistory event mirrors) + 11 AddIndex (10 single-column + 1 composite).

No AlterModelOptions, no RemoveField/AddField. The display_date AlterField in the blog migration is a benign side-effect — pre-existing model/migration drift (model declares db_index=True, but migration 0009 had dropped it) that makemigrations legitimately catches up.

Why this over #921

Just creating the indexes as done in #921 will keep us dependent on TimeStampedModel forever, with the band-aid of manually adding indexes to the only fields the TimeStampedModel adds. We should manually create the fields and remove the inheritance.
— @pablodiegoss, #891 comment

Test plan

• make migrations generates expected operations only
• makemigrations --dry-run --check → "No changes detected" (idempotent)
• Full pytest suite passes (233 tests on SQLite)
• TODO before merge: sqlmigrate core 0028 against Postgres confirms AlterField is metadata-only (no table rewrite). Both CreationDateTimeField and DateTimeField deconstruct to timestamp with time zone, so this should be a no-op.
• TODO before merge: EXPLAIN ANALYZE SELECT * FROM core_exhibit WHERE exhibit_type='AR' ORDER BY created DESC LIMIT 10; against staging confirms Index Scan using exhibit_type_created_idx.
• TODO before merge: Admin smoke test — /admin/blog/post/, /admin/core/exhibit/, /admin/core/sound/ render with created/modified columns and -created ordering.
• Run the migration in a low-traffic window (11 new indexes; brief AddIndex locks).

Related

• Closes Performance: Missing Database Indexes #891
• Supersedes Add indexes on created/modified for timestamped models (#891) #921

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