Drop the pooler from the MVP: point everything straight at Postgres

.env.example and the README still told people to use pgCat on :6432, the
exact setup that hangs on every parameterised query. With no prod and one
backend instance there is nothing for a pooler to multiplex, and
postgres-js already pools in-process, so DATABASE_URL now points at
Postgres on :5432 in .env.example, dev-env.sh and drizzle.config.ts.

prepare:false is kept in src/db/index.ts on purpose, so adding
PgBouncer later (tested working) is a config change rather than a code
change. The long measurement narrative in that file's comment is cut to
the decision and the pointer to PLAN.md, where the numbers now live.

docker-compose.yml

@@ -6,9 +6,8 @@
 # (`ioredis` here, the `redis` crate in continuum-ai-worker) are unchanged.
 # See continuum-common/docs/LICENSES.md.
 #
-# pgCat/PgBouncer is deliberately not here - DATABASE_URL in .env.example
-# points at :6432 for production, but talking straight to Postgres on :5432
-# is one less moving part locally.
+# No pooler anywhere for the MVP: DATABASE_URL points straight at Postgres on
+# :5432 here and in .env.example. See src/db/index.ts for why not pgCat.
 
 services:
   valkey: