i18n: generate messages.pot and enforce EN/ID/POT key parity #694

Parent epic: #679

Evidence

At audit time en.po and id.po contained 827 message IDs while messages.pot contained 623, leaving 204 catalog keys outside the template.

Objective

Make extraction reproducible and prevent missing/stale translations across source, POT, English, and Indonesian catalogs.

Scope

• Add deterministic extraction/generation for messages.pot.
• Add key and placeholder/plural parity checks for EN/ID/POT.
• Define obsolete-entry handling and translator comments.
• Integrate the parity gate into bun run check and CI.
• Document the contributor translation workflow.

Acceptance criteria

• POT contains every active extracted message ID.
• EN and ID have identical active keys and compatible placeholders/plurals.
• Generated output is deterministic.
• Test fixtures prove missing key, placeholder mismatch, and plural mismatch fail.
• Existing locale fallback behavior remains documented and tested.