docs(relationship): ADR-044, DB diagrams, deploy runbook, RTM rows

- ADR-044 extends ADR-039 to the relationship edge: LocalDate+DatePrecision,
  update re-validation of create invariants, no @Version (last-write-wins),
  DELETE→404 anti-enumeration alignment, precise derived marriage date, and the
  relationshipDates.ts location reusing the existing person→shared boundary.
- db-orm.puml: person_relationships now carries from_date/from_date_precision/
  to_date/to_date_precision; db-relationships.puml gets a V78 columns-only note.
- DEPLOYMENT.md §5: V78 deploy note — no maintenance window, stop-old-then-start
  ordering (not rolling-deploy-safe), targeted pg_restore rollback.
- CLAUDE.md error-code list gains INVALID_RELATIONSHIP_DATES.
- rtm.md: REQ-001..REQ-019 for #837 mapped to impl + tests, all Done.

Refs #837
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/DEPLOYMENT.md

@@ -538,6 +538,29 @@ pg_restore -t persons -d ${POSTGRES_DB} backup-YYYYMMDD.dump
 
 (For a plain-SQL dump, extract the persons COPY block instead of replaying the full file.)
 
+### Deploy note — V78 (person_relationships from/to → date + precision, #837)
+
+V78 drops `person_relationships.from_year`/`to_year` after backfilling the new
+`from_date`/`to_date` + precision columns — a **one-way migration** (Flyway cannot roll
+it back). Like V76 it runs its pre-check + DDL in one atomic Flyway transaction and
+needs **no maintenance window** (single-writer archive, no concurrent importers).
+
+It is, however, **not rolling-deploy-safe**: the previously-running JAR still maps the
+`from_year`/`to_year` columns, so it would error against the migrated schema. Deploy in
+this order (the default stop-then-start, single-instance deploy already satisfies it):
+
+1. Take a manual `pg_dump` (see above) and confirm it completed.
+2. **Stop the old JAR**, then **start the new JAR** — Flyway V78 runs first thing on the
+   new JAR's startup, before any request is served. Never run the old and new JARs
+   concurrently across this migration.
+
+If post-deploy data issues are found, restore **only the person_relationships table**
+from the pre-migration dump:
+
+```bash
+pg_restore -t person_relationships -d ${POSTGRES_DB} backup-YYYYMMDD.dump
+```
+
 ### Rollback
 
 Each release tag corresponds to a docker image tag on the host daemon (built via DooD; no registry). Rolling back to a previous tag is one command: