refactor: add foreign key constraint from feature tables to records

[rorybyrne]

Summary

Feature tables (created dynamically in the features PG schema) currently use a plain record_srn TEXT column to link back to records.srn. This should be an actual foreign key constraint.

Why

• pg_graphql (and similar tools) auto-discover relationships via FK constraints — no need for comment directives
• Referential integrity enforced at the DB level
• Our insert volumes don't warrant avoiding FK overhead

Changes

• In PostgresFeatureStore.create_table, change the record_srn column definition to include REFERENCES records(srn)
• Verify bulk insert performance is acceptable with the constraint
• Add migration for any existing feature tables (if any exist in deployed environments)