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