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.

.env.example

@@ -1,7 +1,7 @@
 PORT=3000
 
-# Postgres, routed through pgCat/PgBouncer
-DATABASE_URL=postgres://continuum:continuum@localhost:6432/continuum
+# Postgres, directly - no pooler for the MVP (see src/db/index.ts)
+DATABASE_URL=postgres://continuum:continuum@localhost:5432/continuum
 DATABASE_POOL_MAX=10
 
 # Valkey (state + streams), NOT Redis - Redis 7.4+ is RSALv2/SSPL and 8.0+