Add newsletter translation with NEWSLETTER_LANGUAGES

[kevinhermawan]

Summary

Adds a post-assembly translation step: after each English newsletter is assembled, every language in NEWSLETTER_LANGUAGES is translated by a new translator agent and archived in a new newsletter_translations table. English stays the only language emailed. A translation failure is logged but never blocks the English send.

Also moves beats, campaign, and orchestrator from src/agents/ into a new src/newsroom/ package, keeping agent definitions in src/agents/ and orchestration logic in src/newsroom/.

Related issues

Closes #9

Changes

• src/newsroom/ — new package with beats, campaign, orchestrator, and translation (the translation orchestration step moved from src/agents/)
• src/agents/translator.py — new translator agent via make_agent()
• src/agents/skills/translation/SKILL.md — tone and fidelity rules for the translator
• src/db/newsletter_translations.py — NewsletterTranslation model and upsert_translation/get_translation helpers
• alembic/versions/0002_newsletter_translations.py — migration for the new table
• src/db/newsletters.py — status string changed from "complete" to "completed" for consistency
• Tests, docs, and .env.example updated to match

How to test

1. alembic upgrade head — should apply 0002_newsletter_translations cleanly.
2. pytest — all 142 tests should pass.
3. Set NEWSLETTER_LANGUAGES=Indonesian and run python src/app.py run; check newsletter_translations in Postgres for a row per delivered ticker.