Added `DATABASE_STRUCTURE_CONTEXT.md` to provide detailed
documentation of the database structure for the envelope
domain. This includes entity-to-table mappings, schema
snapshots, EF relationships, constraints, stored procedure
behavior, and trigger wiring paths.
Highlighted model-to-database drift risks, trigger mismatch
alerts, and provided migration-safe operating rules. Added a
refresh checklist to validate metadata consistency before and
after database migrations.
