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

data-pipeline/.env.example

@@ -0,0 +1,7 @@
+# OpenRouter API key — required for OpenRouter providers
+# Get one at https://openrouter.ai/keys
+OPENROUTER_API_KEY=
+
+# Anthropic API key — required for Anthropic provider (reference baseline only)
+# Get one at https://console.anthropic.com/
+ANTHROPIC_API_KEY=