feat(observability): create grafana_reader read-only DB role

Add Flyway V68 migration that provisions a read-only PostgreSQL role
scoped to audit_log, documents, and transcription_blocks. The role's
password is injected via the new ${grafanaDbPassword} Flyway placeholder,
which FlywayConfig reads from the GRAFANA_DB_PASSWORD env var. The
migration is idempotent: CREATE on first run, ALTER on re-run.

Adds a Testcontainers integration test asserting positive grants on the
three intended tables and a negative grant on app_users (NFR-SEC-01).

Refs #651.

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

backend/src/test/java/org/raddatz/familienarchiv/config/GrafanaReaderRoleIntegrationTest.java

@@ -0,0 +1,47 @@
+package org.raddatz.familienarchiv.config;
+
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.PostgresContainerConfig;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.data.jpa.test.autoconfigure.DataJpaTest;
+import org.springframework.boot.jdbc.test.autoconfigure.AutoConfigureTestDatabase;
+import org.springframework.context.annotation.Import;
+import org.springframework.jdbc.core.JdbcTemplate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+@DataJpaTest
+@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
+@Import({PostgresContainerConfig.class, FlywayConfig.class})
+class GrafanaReaderRoleIntegrationTest {
+
+    @Autowired JdbcTemplate jdbc;
+
+    @Test
+    void grafana_reader_has_select_on_audit_log() {
+        assertThat(hasSelect("audit_log")).isTrue();
+    }
+
+    @Test
+    void grafana_reader_has_select_on_documents() {
+        assertThat(hasSelect("documents")).isTrue();
+    }
+
+    @Test
+    void grafana_reader_has_select_on_transcription_blocks() {
+        assertThat(hasSelect("transcription_blocks")).isTrue();
+    }
+
+    @Test
+    void grafana_reader_has_no_select_on_app_users() {
+        assertThat(hasSelect("app_users")).isFalse();
+    }
+
+    private boolean hasSelect(String table) {
+        Boolean result = jdbc.queryForObject(
+                "SELECT has_table_privilege('grafana_reader', ?, 'SELECT')",
+                Boolean.class,
+                table);
+        return Boolean.TRUE.equals(result);
+    }
+}