architecture: persistence (PostgreSQL) and SQL schema

[croadfeldt]

UC map — Realizes UC-1 (persistence + SQL schema).

Implements UDLM contracts: dcm-project/udlm#18 dcm-project/udlm#22 dcm-project/udlm#23 dcm-project/udlm#24 — identifier-scheme + the entity model persisted.
Depends on the UDLM substrate landing first (Wave C); wave index: dcm-project/udlm#35.

Decision: Postgres mandate plus implementation and the initial SQL schema.

Part of the staged croadfeldt/dcm → dcm-project/dcm upstreaming: single-concern PRs, ≤3k lines, logical boundaries. (1298 lines, 3 files.)

Draft — pending team re-engagement.

[croadfeldt]

Superseded by the ordered, scoped set (#109–#117), tracked in #108 — croadfeldt/dcm republished at parity in smaller, self-reviewable PRs; the eng-review feedback carries forward (see #108 and the UDLM carry-forward register). Nothing here is lost.