Use Valkey instead of Redis

Redis 7.4+ is RSALv2/SSPL and 8.0+ adds AGPLv3. None of the three works
for a closed-source commercial cloud service, and SSPL section 13
specifically targets offering the software's functionality as a service,
which is what a control plane does. Valkey is BSD-3-Clause, under the
Linux Foundation, forked from Redis 7.2.4.

Deciding this now rather than after step 3 of PLAN.md stands up real
infrastructure makes it free: Valkey speaks the same RESP protocol, so
`ioredis` is untouched and only names change. `REDIS_URL` becomes
`VALKEY_URL`, the exported client becomes `valkey`, and `/health`
reports a `valkey` field.

Two things deliberately do not change, documented in place so they are
not "fixed" later: the `redis://` URL scheme, which names the wire
protocol and is what every client actually parses (none understands
`valkey://`), and `ioredis` itself, which is a RESP client whose name
refers to the protocol's origin and not to the server we run.

docker-compose.yml is new, pinning valkey/valkey:8-alpine alongside
pgvector/pgvector:pg17 - the plain postgres image lacks the extension
`frame_embeddings` needs. It makes the Valkey choice concrete rather
than a note someone has to remember.

docker-compose.yml

@@ -0,0 +1,45 @@
+# Local development dependencies for continuum-backend.
+#
+# Valkey rather than Redis on purpose: Redis 7.4+ is RSALv2/SSPL and 8.0+
+# adds AGPLv3, none of which a closed-source commercial product can ship.
+# Valkey is BSD-3-Clause and speaks the same RESP protocol, so both clients
+# (`ioredis` here, the `redis` crate in continuum-ai-worker) are unchanged.
+# See continuum-common/docs/LICENSES.md.
+#
+# pgCat/PgBouncer is deliberately not here - DATABASE_URL in .env.example
+# points at :6432 for production, but talking straight to Postgres on :5432
+# is one less moving part locally.
+
+services:
+  valkey:
+    image: valkey/valkey:8-alpine
+    ports:
+      - "6379:6379"
+    command: ["valkey-server", "--appendonly", "yes"]
+    volumes:
+      - valkey-data:/data
+    healthcheck:
+      test: ["CMD", "valkey-cli", "ping"]
+      interval: 5s
+      retries: 5
+
+  postgres:
+    # pgvector/pgvector ships the extension prebuilt; the plain postgres
+    # image does not, and `frame_embeddings` needs it.
+    image: pgvector/pgvector:pg17
+    ports:
+      - "5432:5432"
+    environment:
+      POSTGRES_USER: continuum
+      POSTGRES_PASSWORD: continuum
+      POSTGRES_DB: continuum
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U continuum -d continuum"]
+      interval: 5s
+      retries: 5
+
+volumes:
+  valkey-data:
+  postgres-data: