feat: add core database schema migrations

[Lewiskunta]

Implements: complete database schema for Ratify.

Creates 16 tables in dependency order:
organizations, teams, users, team_members, api_keys,
database_connections, contracts, contract_versions,
contract_columns, consumer_registrations, proposals,
proposal_changes, consumer_responses, breaches,
audit_events, notification_logs

Design decisions:

• All primary keys: UUID with gen_random_uuid()
• All timestamps: TIMESTAMPTZ (UTC, timezone-aware)
• All foreign keys: explicit REFERENCES constraints
• Indexes on all FK columns to prevent slow JOIN queries
• audit_events has no updated_at , append-only by design
• Partial unique index on contracts enforces one active contract per table per connection
• Encrypted fields stored as TEXT (AES-256-GCM output)
• JSONB for schema snapshots, constraint definitions, change values, structured but schema-flexible

Verified:

• Both migrations apply cleanly on docker compose up
• Down migration drops all 16 tables cleanly
• Re-applying up migration restores everything