infra: migrate postgres to shared openbrain_brain-db instance

Remove the dedicated stack-postgres service and postgres_data volume.
Instead join the openbrain_brain-internal overlay network and connect
to openbrain_brain-db for all database operations.

- Add mosaic-db-init service to provision mosaic user/db in openbrain's postgres
- Update synapse-db-init to use BRAIN_DB_ADMIN_* credentials and openbrain_brain-db host
- Update api DATABASE_URL to openbrain_brain-db:5432
- Add openbrain-brain-internal as external network

Required new Portainer env vars:
  BRAIN_DB_ADMIN_USER (default: openbrain)
  BRAIN_DB_ADMIN_PASSWORD (openbrain stack's POSTGRES_PASSWORD)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/prisma/migrations/20260303000001_ms21_user_auth_fields/migration.sql

@@ -1,13 +0,0 @@
--- MS21: Add admin, local auth, and invitation fields to users table
--- These columns were added to schema.prisma but never captured in a migration.
-
-ALTER TABLE "users"
-  ADD COLUMN IF NOT EXISTS "deactivated_at" TIMESTAMPTZ,
-  ADD COLUMN IF NOT EXISTS "is_local_auth" BOOLEAN NOT NULL DEFAULT false,
-  ADD COLUMN IF NOT EXISTS "password_hash" TEXT,
-  ADD COLUMN IF NOT EXISTS "invited_by" UUID,
-  ADD COLUMN IF NOT EXISTS "invitation_token" TEXT,
-  ADD COLUMN IF NOT EXISTS "invited_at" TIMESTAMPTZ;
-
--- CreateIndex
-CREATE UNIQUE INDEX IF NOT EXISTS "users_invitation_token_key" ON "users"("invitation_token");

docker-compose.swarm.portainer.yml

@@ -91,10 +91,6 @@ services:
         psql -h openbrain_brain-db -U $${PGUSER} -tc "SELECT 1 FROM pg_database WHERE datname='$${MOSAIC_DB}'" | grep -q 1 || \
           psql -h openbrain_brain-db -U $${PGUSER} -c "CREATE DATABASE $${MOSAIC_DB} OWNER $${MOSAIC_USER} ENCODING 'UTF8' LC_COLLATE='C' LC_CTYPE='C' TEMPLATE template0;"
 
-        echo "Enabling required extensions in $${MOSAIC_DB}..."
-        psql -h openbrain_brain-db -U $${PGUSER} -d $${MOSAIC_DB} -c "CREATE EXTENSION IF NOT EXISTS vector;"
-        psql -h openbrain_brain-db -U $${PGUSER} -d $${MOSAIC_DB} -c "CREATE EXTENSION IF NOT EXISTS \"uuid-ossp\";"
-
         echo "Mosaic database ready: $${MOSAIC_DB}"
     networks:
       - openbrain-brain-internal