perf(datasource): reuse query plans on core connections (#49)

Planning was 94% of Dashboard DB time, and prod never reused a plan.
Force generic plans and prepare on first use. Burst: 443 -> 229 ms.

readme.md

@@ -909,6 +909,14 @@ duration, e.g. `GET /api/library/games 200 28 ms`. The pattern is `-` when no ha
 routing). The line never holds ids, query strings, user data or tokens. `/actuator/**` is skipped. An exception
 that leaves the filter chain is logged as `500`.
 
+### Database connection settings
+
+Every Core connection runs with `plan_cache_mode = force_generic_plan` and `prepareThreshold=1`
+(`spring.datasource.hikari` in `application.yml`). Planning was 94% of the Dashboard DB time and prod never reused a
+plan (few users, a new connection every 30 min). With a cached generic plan the Dashboard burst dropped from 443 to
+229 ms on 2 CPUs (#49). Cost: the first burst on fresh connections costs about 100 ms more (cold start: 1101 -> 1205
+ms), and a generic plan ignores parameter values.
+
 ## Tests
 
 ```bash

src/main/resources/application.yml

@@ -8,6 +8,12 @@ spring:
       accept-float-as-int: false
   jpa:
     open-in-view: false
+  datasource:
+    hikari:
+      # Plan once, not per request: Dashboard burst 443 -> 229 ms, 94% of the DB time was planning (#49).
+      data-source-properties:
+        prepareThreshold: 1
+      connection-init-sql: set plan_cache_mode = force_generic_plan
   flyway:
     enabled: true
     baseline-on-migrate: true

src/test/java/core_service/configuration/DataSourceSettingsTest.java

@@ -0,0 +1,28 @@
+package core_service.configuration;
+
+import core_service.shared.test.AbstractPersistenceTest;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.jdbc.core.JdbcTemplate;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class DataSourceSettingsTest extends AbstractPersistenceTest {
+
+    @Autowired
+    private JdbcTemplate jdbc;
+
+    @Test
+    void coreConnection_ForcesGenericPlans() {
+        assertThat(jdbc.queryForObject("show plan_cache_mode", String.class)).isEqualTo("force_generic_plan");
+    }
+
+    @Test
+    void coreConnection_PreparesOnTheFirstUse() {
+        jdbc.queryForObject("select 1 where ? = 1", Integer.class, 1);
+
+        assertThat(jdbc.queryForObject(
+                "select count(*) from pg_prepared_statements where statement like 'select 1 where%'", Integer.class))
+                .isEqualTo(1);
+    }
+}