chore(docs+tests): reactivate EntriesIsolationIntegrationTest and resolve tenant data isolation issues

- Fixed schema isolation handling in Exposed by switching table creation to JDBC and explicitly setting `search_path` in PostgreSQL.
- Removed redundant `runBlocking` calls, unused variables, and IDE warnings in the test.
- Added `JwtDecoder` mock in `@TestConfiguration` to prevent application context loading errors.
- Verified that writes in one tenant schema are no longer visible in another.

chore(config): add `application-test.yaml` for better test environment setup

- Configured H2 as an in-memory database for tests.
- Disabled Flyway and Consul to avoid unnecessary dependencies during testing.

backend/services/entries/entries-service/src/test/resources/application-test.yaml

@@ -0,0 +1,24 @@
+spring:
+  datasource:
+    url: jdbc:h2:mem:entries-test;DB_CLOSE_DELAY=-1;MODE=PostgreSQL
+    driver-class-name: org.h2.Driver
+    username: sa
+    password:
+  flyway:
+    enabled: false
+  cloud:
+    consul:
+      enabled: false
+      discovery:
+        enabled: false
+  security:
+    oauth2:
+      resourceserver:
+        jwt:
+          issuer-uri: http://localhost:8180/realms/meldestelle
+          jwk-set-uri: http://localhost:8180/realms/meldestelle/protocol/openid-connect/certs
+
+# Multi-tenancy settings for tests
+multitenancy:
+  registry:
+    type: inmem