feat(infra): docker-compose.federated.yml overlay for federated tier (FED-M1-02)

Adds a profile-gated compose overlay defining `postgres-federated`
(pgvector/pgvector:pg17, port 5433) and `valkey-federated`
(valkey/valkey:8-alpine, port 6380) with named volumes
(`pg_federated_data`, `valkey_federated_data`), healthchecks identical
to the base stack, and the existing `infra/pg-init` mount so the vector
extension is created automatically on first boot.

Both services are gated by `profiles: [federated]` so they never start
on a plain `docker compose up`. Usage:

    docker compose -f docker-compose.federated.yml --profile federated up -d

The overlay is mutually exclusive with the base dev stack on host ports
5433/6380 (header comment documents this). Base file untouched.

Refs #460

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.federated.yml

@@ -0,0 +1,60 @@
+# docker-compose.federated.yml — Federated tier overlay
+#
+# USAGE:
+#   docker compose -f docker-compose.federated.yml --profile federated up -d
+#
+# This file is a standalone overlay for the Mosaic federated tier.
+# It is NOT an extension of docker-compose.yml — it defines its own services
+# and named volumes so it can run independently of the base dev stack.
+#
+# IMPORTANT — HOST PORT CONFLICTS:
+#   The federated services bind the same host ports as the base dev stack
+#   (5433 for Postgres, 6380 for Valkey). You must stop the base dev stack
+#   before starting the federated stack on the same machine:
+#     docker compose down
+#     docker compose -f docker-compose.federated.yml --profile federated up -d
+#
+# pgvector extension:
+#   The vector extension is created automatically at first boot via
+#   ./infra/pg-init/01-extensions.sql (CREATE EXTENSION IF NOT EXISTS vector).
+#
+# Tier configuration:
+#   Used by `mosaic` instances configured with `tier: federated`.
+#   DEFAULT_FEDERATED_CONFIG points at:
+#     postgresql://mosaic:mosaic@localhost:5433/mosaic
+
+services:
+  postgres-federated:
+    image: pgvector/pgvector:pg17
+    profiles: [federated]
+    ports:
+      - '${PG_FEDERATED_HOST_PORT:-5433}:5432'
+    environment:
+      POSTGRES_USER: mosaic
+      POSTGRES_PASSWORD: mosaic
+      POSTGRES_DB: mosaic
+    volumes:
+      - pg_federated_data:/var/lib/postgresql/data
+      - ./infra/pg-init:/docker-entrypoint-initdb.d:ro
+    healthcheck:
+      test: ['CMD-SHELL', 'pg_isready -U mosaic']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  valkey-federated:
+    image: valkey/valkey:8-alpine
+    profiles: [federated]
+    ports:
+      - '${VALKEY_FEDERATED_HOST_PORT:-6380}:6379'
+    volumes:
+      - valkey_federated_data:/data
+    healthcheck:
+      test: ['CMD', 'valkey-cli', 'ping']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+volumes:
+  pg_federated_data:
+  valkey_federated_data: