Move the browser off the VPS to its own unit (#46)

The headless browser leaves the API stack. It becomes its own compose unit
(chrome/docker-compose.yml) deployed on the home machine and reached over the
tailnet, returning 471 MiB of working set to a 1974 MiB VPS that has no swap.
No fallback sidecar is left behind.

The backend needs no code change: BROWSER_WS_URL was already the only coupling,
so relocation is one environment variable. Its default is now empty rather than
a pinned Docker IP — an unreachable or unconfigured browser degrades exactly as
it always has, with plain-TLS libraries unaffected, kagane and novelfull logged
and skipped, and stored covers still served.

The browser unit publishes CDP on ${BROWSER_BIND_ADDR} with no default, because
CDP authenticates nothing and the home machine has a real LAN: an unset value
must fail the deploy rather than silently expose an endpoint that is remote code
execution for anything that reaches it. Resource limits are sized against the
measured 645 MiB untuned peak and the CI runner that already holds 1.2 GiB of
that box.

bookmark-api gains the default network. Dropping `browser` left it on `db`
alone, which is internal: true — that meant no published port and, worse, no
egress for the poller at all. Caught by bringing the stack up.

Docs: ADR-0006 for the topology, DEPLOY.md §7 for first-time setup of the
browser machine, REDEPLOY.md §8 for its independent update cadence, plus the
architecture diagrams, config tables and troubleshooting rows.

docker-compose.prod.yml

@@ -17,23 +17,13 @@ services:
   bookmark-api:
     # Traffic arrives over the Traefik network, not a published port.
     ports: !reset []
-    environment:
-      # Must be an IP, not the DNS name — see the base file's comment on this
-      # same key: Chrome's DevTools HTTP handler 500s any Host header that
-      # isn't an IP or "localhost".
-      BROWSER_WS_URL: ${BROWSER_WS_URL:-ws://172.28.0.10:9222}
-    depends_on:
-      headless-shell:
-        condition: service_started
-      postgres:
-        condition: service_healthy
-    # `networks:` here replaces the base file's list entirely, so all three must
-    # be named: `proxy` for Traefik routing, and `browser` / `db` (defined in
-    # the base file) to keep reaching headless-shell and Postgres without
-    # putting either on `proxy`.
+    # `networks:` here replaces the base file's list entirely, so both must be
+    # named: `proxy` for Traefik routing, and `db` (defined in the base file)
+    # to keep reaching Postgres without putting it on `proxy`. `proxy` also
+    # carries the poller's outbound traffic — `db` is `internal: true`, so a
+    # container on it alone has no egress at all.
     networks:
       - proxy
-      - browser
       - db
     labels:
       - "traefik.enable=true"
@@ -52,10 +42,12 @@ services:
       - "traefik.http.routers.bmweb.tls.certresolver=${TRAEFIK_CERTRESOLVER:-le}"
       - "traefik.http.routers.bmweb.service=bmapi"
 
-  # headless-shell is untouched here: it keeps its `browser` network membership
-  # from the base file and must never join `proxy` — that network is shared
-  # with whatever else sits behind Traefik on this host, and an exposed
-  # CDP endpoint on it would be remote code execution for any of them.
+  # No browser service here. It runs on the home machine as its own unit
+  # (chrome/docker-compose.yml) and is reached over the tailnet — see
+  # docs/adr/0006-browser-on-the-home-machine.md. It must never be given a
+  # service on this host: `proxy` is shared with whatever else sits behind
+  # Traefik, and an unauthenticated CDP endpoint on it is remote code
+  # execution for any of them.
 
 networks:
   proxy: