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.

README.md

@@ -1,7 +1,7 @@
 # continuum-ai-worker
 
 Computer-vision failure detection worker for the Continuum print farm
-platform. Consumes camera frame batches pushed onto the Redis Stream
+platform. Consumes camera frame batches pushed onto the Valkey Stream
 `stream:camera_frames` (by `continuum-backend`'s `src/queue/`) and reports
 failures.
 
@@ -10,8 +10,8 @@ failures.
 Real ONNX Runtime inference (the `ort` crate) and the Postgres audit write
 (`sqlx`) are **not** implemented yet - `src/inference/mod.rs` always
 "detects" one fixed example failure, and `src/reporter/mod.rs` only prints
-where the Postgres write would go. The Redis Streams consumer-group loop in
-`main.rs` and the Redis pub/sub alert in `reporter/` are real. This lets the
+where the Postgres write would go. The Valkey Streams consumer-group loop in
+`main.rs` and the Valkey pub/sub alert in `reporter/` are real. This lets the
 whole pipeline (read a frame, "detect", report, ack) run and be understood
 before adding a real model or a SQL database on top.
 
@@ -24,7 +24,7 @@ continuum-proxy --(frames)--> continuum-backend --(XADD)--> stream:camera_frames
                                                                     v
                                                      inference::detect (stub for now)
                                                                     v
-                                                 reporter::report (Redis pub/sub - real)
+                                                 reporter::report (Valkey pub/sub - real)
 ```
 
 ## Getting started
@@ -41,5 +41,5 @@ src/
   main.rs          Consumer-group loop: XREADGROUP -> detect -> report -> XACK
   config.rs        Loads settings from environment variables
   inference/       Detection stub - real ONNX Runtime inference goes here later
-  reporter/        Redis pub/sub alert (real) + a note for the Postgres write (later)
+  reporter/        Valkey pub/sub alert (real) + a note for the Postgres write (later)
 ```