Add indexes on created/modified for timestamped models (#891)

[vjpixel]

Declare Meta.indexes on all 8 TimeStampedModel-inheriting models (Sound, Marker, Object, Artwork, Exhibit, Post, PostImage, Clipping) so makemigrations produces standard AddIndex operations on -created and -modified. Adds a composite (exhibit_type, -created) index on Exhibit for the filter+order pattern in user profile views. No changes to the django-extensions inheritance.

Closes #891

https://claude.ai/code/session_01XC1THLWgnGXGf5wgRhdyvB

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[vjpixel]

Self-review

Verdict: ✅

Declares Meta.indexes on all 8 TimeStampedModel-inheriting models. makemigrations produces clean AddIndex operations only — no AlterField/AlterModelOptions noise. The composite (exhibit_type, -created) index covers the filter+order pattern in users/views.py:109,112 (Exhibit.objects.filter(exhibit_type=AR/MR).order_by("-created")).

Choices documented:

• Indexed both -created and -modified (per user direction). All ModelAdmin classes use ordering = ["-created"]; blog models use get_latest_by = "modified" for .latest() calls.
• Standard AddIndex (not AddIndexConcurrently) — simpler, takes a brief table lock during creation. Acceptable for current data volume per user direction. Switch to concurrent if scaling later.
• Kept TimeStampedModel inheritance and django-extensions dependency intact, contrary to the issue's original suggestion.

Verified:

• make migrations --check --dry-run reports no further changes after the migration files are committed.
• make migrate applies cleanly against SQLite (and prod is PostgreSQL).
• 107 tests still pass locally; the rest fail on environmental file paths (docker mount), unrelated.

Unique: No duplicate PR.

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