Speed up test suite and remove unused backfill command

[bpepple]

Summary

• Test suite was paying the full production-strength PBKDF2 cost (~0.5s/hash) on every user creation, and fixture chains often create several users per test (metadata fixtures each create their own created_by/edited_by user), making this the dominant cost by far. Switch to MD5PasswordHasher under pytest only — verified the override is inert outside pytest and that pytest isn't installed in a production (--no-dev) sync, so production hashing is unaffected.
• Add --reuse-db to skip rebuilding the test database from ~103 migrations on every invocation; safe in CI since each job starts from a fresh Postgres container. Documented the --create-db escape hatch for local migration changes.
• Remove issue_ratings.backfill_issue_ratings, a one-time management command that already did its job, along with its tests and an unused other_rating_user test fixture.

Together the two speed changes cut the full suite from ~13m40s to ~51s locally (same 1780 passed / 2 skipped).

Test plan

• Verified production isn't affected: PASSWORD_HASHERS is only set when "pytest" in sys.modules, and confirmed pytest is excluded from a uv sync --no-dev install