fix: preserve embedding outage status for trigger endpoints

[JulianZJN]

Closes #137

Summary

• Move the shared embedding-unavailable 503 helper into palinode/api/_util.py and reuse it from the search and trigger routers.
• Catch EmbeddingUnavailable before the generic 500 handler in /triggers and /check-triggers.
• Leave the existing empty-embedding branches unchanged and add one regression test per endpoint.

Tests

• uv run pytest tests/test_search_embedder_outage.py tests/test_triggers_vec_upsert.py -q (15 passed)
• uv run ruff check palinode/ tests/ scripts/
• uv run bandit -r palinode/ -ll (no issues identified)
• git diff --check