feat(ops): vision-manager sidecar to autoscale the llama.cpp container

Add a tiny privileged sidecar (vision-manager/) that polls the analysis
backlog in Postgres and starts/stops the mangalord-vision container by name:
start when analyze_page jobs are pending, stop after STOP_DEBOUNCE idle.
It is the single owner of the vision lifecycle and the only component with
Docker access — scoped through tecnativa/docker-socket-proxy (CONTAINERS+POST
only) on an internal-only network, so the internet-facing backend never
touches the socket.

Both helpers sit behind `profiles: [ai]` (vanilla `compose up` is
unaffected). The manager debounces the stop, gates the first request on
GET /health==200 after a cold start, honours a crawl RAM-mutex on the 8 GiB
box, and owns its own idle timer (leak-safe if the backend dies). Backlog
query uses the real schema (state + payload->>'kind'); a read-only DB role
(readonly-role.sql) keeps it off the backend creds.

Bump 0.80.0 -> 0.81.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.80.0"
+version = "0.81.0"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.80.0"
+version = "0.81.0"
 edition = "2021"
 default-run = "mangalord"