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.

backend/AGENTS.md

@@ -128,10 +128,13 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `/userscript/novel-bookmark.user.js`, both supplied by bindmount; the
   `__API_TOKEN__` placeholder inside them is substituted with the requesting
   Reader's credential at serve time).
-  `BROWSER_WS_URL` (CDP endpoint of the `chrome/` sidecar, used by the poller
-  for kagane and novelfull *and* by the web UI's kagane cover proxy; unset
-  disables browser polling and serves 404 from the proxy, leaving those sites
-  to the userscript alone).
+  `BROWSER_WS_URL` (CDP endpoint of the browser, which runs on a **separate
+  machine** and is reached over the tailnet — ADR-0006, `chrome/docker-compose.yml`.
+  Used by the poller for kagane and novelfull *and* by the web UI's kagane
+  cover proxy; unset — the default — disables browser polling and serves 404
+  for covers not already stored, leaving those sites to the userscript alone.
+  Must be a tailnet IP, never a hostname: Chrome's DevTools handler 500s
+  `/json/version` for any Host that isn't an IP or `localhost`).
 - **kagane covers are proxied, not hot-linked:** kagane serves cover images
   behind the same challenge as its pages and with
   `cross-origin-resource-policy: same-origin`, so no `<img>` on the web UI's