feat: enrich stage foundation — provider config, env setup, schema fix

- Remove foreign key on run_status.source_id to support sentinel rows
  for tracking one-time pipeline steps (compile_candidates, compile_votes,
  merge, compare)
- Add stage-3-enrich/config.ts with all provider configurations,
  ALL_PROVIDERS ordered local-first, and validateProviderKey() for
  startup key checks
- Add .env.example with required API keys for OpenRouter and Anthropic
- Add pipeline:run script to package.json using --env-file .env
- Add .env to root .gitignore coverage for data-pipeline/.env

.gitignore

@@ -17,4 +17,4 @@ data-pipeline/stage-3-enrich/output/
 data-pipeline/stage-4-merge/output/
 data-pipeline/db/pipeline.db
 data-pipeline/reports/
-
+data-pipeline/.env