selfhost(infra): PgBouncer connection pooler

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Part of #977.

Add a PgBouncer sidecar to the Postgres deployment path. Without pooling, each app instance holds a dedicated Postgres connection; above ~20 concurrent reviews the connection limit becomes the ceiling.

Scope:

• bitnami/pgbouncer service in the pgbouncer compose profile
• Transaction-mode pooling (max_client_conn=200, default_pool_size=20)
• Operator routes DATABASE_URL to pgbouncer:5432 instead of postgres:5432
• SCRAM-SHA-256 authentication passthrough

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Closing as completed (2026-07-01 issue audit): PgBouncer sidecar shipped in docker-compose.yml under the pgbouncer profile.