feat(observability): own grafana_reader password via repeatable migration

V68 used to set the role's password in a versioned migration, which Flyway
applies exactly once per database. Rotating GRAFANA_DB_PASSWORD therefore
had no effect on the DB role — operators would need a manual ALTER ROLE
or a `flyway repair` that nobody documented. The shape conflated two
lifecycles: schema migration (one-shot, immutable) and credential
provisioning (rotatable).

Split into:
- V68 (versioned, immutable): creates the role and applies SELECT grants
  on audit_log, documents, transcription_blocks.
- R__grafana_reader_password.sql (repeatable): issues ALTER ROLE … PASSWORD
  with the placeholder. Flyway computes the checksum on the resolved
  content, so any change to GRAFANA_DB_PASSWORD changes the checksum and
  re-applies the migration on the next boot. Rotation becomes "bump env
  var + restart backend".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/resources/db/migration/R__grafana_reader_password.sql

@@ -0,0 +1,14 @@
+-- Repeatable migration: sets the grafana_reader role's password from the
+-- ${grafanaDbPassword} placeholder (resolved by FlywayConfig from the
+-- GRAFANA_DB_PASSWORD environment variable). Flyway computes the checksum on
+-- the resolved migration content, so any change to GRAFANA_DB_PASSWORD changes
+-- the checksum and re-applies this migration on the next boot. That makes
+-- password rotation a "change env var + restart" operation — no manual psql.
+--
+-- V68 created the role itself (without a usable password). This file owns the
+-- password lifecycle; nothing else writes it.
+DO $$
+BEGIN
+    EXECUTE format('ALTER ROLE grafana_reader WITH PASSWORD %L', '${grafanaDbPassword}');
+END
+$$;

backend/src/main/resources/db/migration/V68__add_grafana_reader_role.sql

@@ -1,13 +1,13 @@
 -- Read-only role used by the Grafana PostgreSQL datasource for the PO Overview
--- dashboard (issue #651). Password is injected at migration time via the Flyway
--- placeholder ${grafanaDbPassword}, supplied by FlywayConfig from the
--- GRAFANA_DB_PASSWORD environment variable.
+-- dashboard (issue #651). The role is created here without a usable password
+-- (LOGIN-capable but no password set); R__grafana_reader_password.sql sets the
+-- password from GRAFANA_DB_PASSWORD on every boot, so rotation is just "bump
+-- the env var and restart the backend" — see docs/adr/024-* and the rotation
+-- runbook in docs/DEPLOYMENT.md.
 DO $$
 BEGIN
     IF NOT EXISTS (SELECT 1 FROM pg_catalog.pg_roles WHERE rolname = 'grafana_reader') THEN
-        EXECUTE format('CREATE ROLE grafana_reader WITH LOGIN PASSWORD %L', '${grafanaDbPassword}');
-    ELSE
-        EXECUTE format('ALTER ROLE grafana_reader WITH LOGIN PASSWORD %L', '${grafanaDbPassword}');
+        CREATE ROLE grafana_reader WITH LOGIN;
     END IF;
 END
 $$;