fix: use postgres-version input consistently in action.yaml

[veksen]

Replace all hardcoded PostgreSQL version references in action.yaml with the postgres-version input. The cache path now targets only the requested version directory (fixing tar spam from caching pre-installed PG 16), the cache key uses the input, apt-get install installs the requested version instead of hardcoded 18/17 fallback, and pg_dump/pg_restore paths use the input directly instead of scanning /usr/lib/postgresql.

[github-actions]

Query Doctor Analysis

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84 queries analyzed

2 pre-existing issues

• SELECT "guests"."id", "guests"."session_id", "guests"."username", "guests"."avatar_path", "guests"."color", "guests"."side", "guests"."audio_recording_path", "guests"."audio_recording_public", "gue...
  index assets(event_id, uploader_id, inserted_at desc)
  cost 15,918 → 1,631 (90% reduction)
• SELECT * FROM guest_ip_addresses WHERE ip_address = '127.0.0.1';
  index guest_ip_addresses(ip_address)
  cost 126 → 9 (93% reduction)

_{Using assumed statistics (10000 rows/table). For better results, sync production stats.}