Use Valkey instead of Redis

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 the `redis` crate works unchanged -
`CONTINUUM_REDIS_URL` becomes `CONTINUUM_VALKEY_URL`, `Config.redis_url`
becomes `valkey_url`, and `Reporter`'s connection field follows.

The `redis://` scheme and the `redis` crate itself deliberately stay:
both name the RESP wire protocol rather than the server we deploy, and
redis-rs has no `valkey://` scheme to use instead. Commented in place so
the inconsistency reads as intentional.

.env.example

@@ -1,4 +1,7 @@
-CONTINUUM_REDIS_URL=redis://localhost:6379
+# Valkey, not Redis (licensing - see continuum-common/docs/LICENSES.md).
+# The `redis://` scheme is the RESP wire protocol; redis-rs has no
+# `valkey://` scheme to use instead.
+CONTINUUM_VALKEY_URL=redis://localhost:6379
 CONTINUUM_FRAME_STREAM=stream:camera_frames
 CONTINUUM_CONSUMER_GROUP=ai-worker
 CONTINUUM_CONSUMER_NAME=ai-worker-1