feat(db): log which connection each dbFor sub-pool resolved to

One line per role at first use: the dedicated DATABASE_URL_<ROLE> when set,
otherwise an explicit fallback message naming the process connection it
shares — so a missing/typo'd env var is visible at rollout instead of silent.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NNy9Fzpfc1FdHAzYyb6Ycy

Author: TheodoreSpeaks

packages/db/db.ts

@@ -1,9 +1,12 @@
+import { createLogger } from '@sim/logger'
 import { drizzle } from 'drizzle-orm/postgres-js'
 import postgres from 'postgres'
 import { resolveDbUrl } from './connection-url'
 import * as schema from './schema'
 import { instrumentPoolClient } from './tx-tripwire'
 
+const logger = createLogger('Db')
+
 /**
  * Per-role pool profiles. Starting numbers — validate against real per-role
  * process counts (PgBouncer transaction mode, max_connections=200).
@@ -81,6 +84,11 @@ export const dbReplica: typeof db = replicaUrl
 
 const subPoolClients = new Map<SubProcessDbRole, typeof db>()
 
+/** Which env var the process connection came from — named in dbFor fallback logs. */
+const processUrlEnvVar = process.env[`DATABASE_URL_${role.toUpperCase()}`]
+  ? `DATABASE_URL_${role.toUpperCase()}`
+  : 'DATABASE_URL'
+
 /**
  * Per-workload drizzle client with its own pool, built lazily on first call and
  * cached per role. Unlike the process-wide `db` (selected by `SIM_DB_ROLE`),
@@ -98,11 +106,21 @@ export function dbFor(role: SubProcessDbRole): typeof db {
   const existing = subPoolClients.get(role)
   if (existing) return existing
 
-  const url = process.env[`DATABASE_URL_${role.toUpperCase()}`] ?? connectionString
+  const keyedEnvVar = `DATABASE_URL_${role.toUpperCase()}`
+  const keyedUrl = process.env[keyedEnvVar]
+  const url = keyedUrl ?? connectionString
   if (!url) {
     throw new Error('Missing DATABASE_URL environment variable')
   }
 
+  if (keyedUrl) {
+    logger.info(`'${role}' pool using dedicated ${keyedEnvVar}`)
+  } else {
+    logger.info(
+      `${keyedEnvVar} not set — '${role}' pool falling back to the process connection (${processUrlEnvVar})`
+    )
+  }
+
   const subProfile = DB_POOL_PROFILES[role]
   const client = drizzle(
     instrumentPoolClient(