feat(w5): dual Flyway history (V1-V6)

Migrations in db/migration/auth/ with separate flyway_schema_history_auth table:
- V1: users + user_identities (with provider/subject unique constraint)
- V2: memberships (polymorphic org_type/org_id, unique per user+org)
- V3: invitations (64-char token, status lifecycle)
- V4: access_requests (requester → reviewer workflow)
- V5: Microsoft tenant_id partial index on user_identities
- V6: login_events + refresh_tokens + revinfo actor_user_id column

PlateAuthFlywayConfig runs a second Flyway bean against flyway_schema_history_auth,
independent of consumer's own flyway_schema_history. Runs at bean init (before JPA).

plate-auth-starter/src/main/java/de/platesoft/auth/config/PlateAuthFlywayConfig.java

@@ -0,0 +1,28 @@
+package de.platesoft.auth.config;
+
+import org.flywaydb.core.Flyway;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+import javax.sql.DataSource;
+
+/**
+ * Configures a separate Flyway instance for plate-auth migrations.
+ * Uses its own history table (flyway_schema_history_auth) to avoid
+ * version collisions with the consumer application's migrations.
+ */
+@Configuration
+@ConditionalOnClass(Flyway.class)
+public class PlateAuthFlywayConfig {
+
+    @Bean(initMethod = "migrate")
+    public Flyway plateAuthFlyway(DataSource dataSource) {
+        return Flyway.configure()
+                .dataSource(dataSource)
+                .locations("classpath:db/migration/auth")
+                .table("flyway_schema_history_auth")
+                .baselineOnMigrate(true)
+                .load();
+    }
+}