feat(observability): fail closed when GRAFANA_DB_PASSWORD is unset

FlywayConfig used to fall back to a hardcoded "changeme-grafana-db-password"
string when the env var was missing. That published a known credential for
the grafana_reader role (SELECT on audit_log, documents, transcription_blocks)
into git history and made silent fail-open the default for any deploy that
forgot the secret. Now resolution goes through Spring's Environment and
throws IllegalStateException at startup when the value is unset or blank —
same shape as UserDataInitializer's refusal to seed default admin creds.

Tests inject via the global GRAFANA_DB_PASSWORD entry in test-resources
application.properties so existing Flyway-loading test classes keep
booting without per-class TestPropertySource boilerplate. FlywayConfigTest
covers both branches against MockEnvironment without a Spring context.

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

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

@@ -0,0 +1,37 @@
+package org.raddatz.familienarchiv.config;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.mock.env.MockEnvironment;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class FlywayConfigTest {
+
+    @Test
+    void resolveGrafanaDbPassword_throws_when_env_unset() {
+        FlywayConfig config = new FlywayConfig(null, new MockEnvironment());
+
+        assertThatThrownBy(config::resolveGrafanaDbPassword)
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("GRAFANA_DB_PASSWORD is required");
+    }
+
+    @Test
+    void resolveGrafanaDbPassword_throws_when_env_blank() {
+        MockEnvironment env = new MockEnvironment().withProperty("GRAFANA_DB_PASSWORD", "   ");
+        FlywayConfig config = new FlywayConfig(null, env);
+
+        assertThatThrownBy(config::resolveGrafanaDbPassword)
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("GRAFANA_DB_PASSWORD is required");
+    }
+
+    @Test
+    void resolveGrafanaDbPassword_returns_value_when_env_set() {
+        MockEnvironment env = new MockEnvironment().withProperty("GRAFANA_DB_PASSWORD", "abc");
+        FlywayConfig config = new FlywayConfig(null, env);
+
+        assertThat(config.resolveGrafanaDbPassword()).isEqualTo("abc");
+    }
+}

backend/src/test/resources/application.properties

@@ -1,2 +1,8 @@
 logging.level.root=WARN
 logging.level.org.raddatz=INFO
+
+# Default test value so FlywayConfig's fail-closed check passes without each
+# test having to set GRAFANA_DB_PASSWORD explicitly. The actual value is
+# irrelevant in tests — Flyway only uses it to set the grafana_reader role's
+# password, which no test connects with.
+GRAFANA_DB_PASSWORD=test-grafana-reader-password