Fix prod deploy Flyway validation for removed dev seed migrations.

Backend crashed on startup because the prod DB still records V6 (and
possibly V2/V4) from when seeds lived in db/migration, while prod only
loads schema migrations. ignore-migration-patterns alone did not prevent
validate failure on the runner.

- Run Flyway repair before migrate on the prod profile
- Add ProdFlywayConfigTest for repair-then-migrate order
- Document the V6 error in README deploy troubleshooting

README.md

@@ -317,11 +317,15 @@ If the job passes the frontend check but the backend never becomes healthy:
 
 1. Open the failed job log and read **Backend logs** (printed before rollback).
 2. Match the error to a fix — do not guess:
+   - **`Detected applied migration not resolved locally: 6`** (or 2, 4) — prod DB still lists
+     dev seed migrations removed from `db/migration`. Fixed in app via `ProdFlywayConfig`
+     (repair before migrate); redeploy after that commit is on `master`.
    - **`password authentication failed`** — DB credentials in the running stack do not match
      what Postgres was initialized with; fix credentials or Postgres password to match (only
      wipe the volume if you accept losing prod data).
    - **`Production requires ADMIN_EMAIL and ADMIN_PASSWORD`** — add those Forgejo secrets.
-   - **Flyway / migration errors** — fix schema or migration history before redeploying.
+   - **Other Flyway / migration errors** — read the stack trace; do not wipe the volume unless
+     the log clearly requires it.
 3. **DBeaver from your laptop** — prod Postgres binds to `127.0.0.1:5433` on the server only.
    Use an SSH tunnel, then host `localhost` port `5433` (not `192.168.0.59` directly).
 

backend/src/main/java/se/bilhalsning/config/ProdFlywayConfig.java

@@ -0,0 +1,24 @@
+package se.bilhalsning.config;
+
+import org.flywaydb.core.Flyway;
+import org.springframework.boot.flyway.autoconfigure.FlywayMigrationStrategy;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Profile;
+
+@Configuration
+@Profile("prod")
+public class ProdFlywayConfig {
+
+    /**
+     * Prod databases may record dev seed migrations (V2, V4, V6) from before they moved to
+     * db/dev-migration/. Repair marks those missing scripts as deleted so validate passes.
+     */
+    @Bean
+    public FlywayMigrationStrategy prodFlywayMigrationStrategy() {
+        return (Flyway flyway) -> {
+            flyway.repair();
+            flyway.migrate();
+        };
+    }
+}

backend/src/test/java/se/bilhalsning/config/ProdFlywayConfigTest.java

@@ -0,0 +1,33 @@
+package se.bilhalsning.config;
+
+import static org.mockito.Mockito.inOrder;
+import static org.mockito.Mockito.mock;
+
+import org.flywaydb.core.Flyway;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.flyway.autoconfigure.FlywayMigrationStrategy;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.test.context.ActiveProfiles;
+import org.mockito.InOrder;
+
+@SpringBootTest(
+        properties = {
+            "app.admin.email=admin@test.se",
+            "app.admin.password=test-password",
+        })
+@ActiveProfiles("prod")
+class ProdFlywayConfigTest {
+
+    @Autowired
+    private FlywayMigrationStrategy flywayMigrationStrategy;
+
+    @Test
+    void shouldRepairBeforeMigrateOnProd() {
+        Flyway flyway = mock(Flyway.class);
+        flywayMigrationStrategy.migrate(flyway);
+        InOrder order = inOrder(flyway);
+        order.verify(flyway).repair();
+        order.verify(flyway).migrate();
+    }
+}