Move the browser off the VPS to its own unit (#46) (#52)

Closes #46 once deployed.

The headless browser leaves the API stack and becomes its own compose unit
(`chrome/docker-compose.yml`) intended for the home machine, reached over the
tailnet. No fallback sidecar is left on the VPS.

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

### What shipped

- `chrome/docker-compose.yml` + `chrome/.env.example` — the browser unit, with
  the CDP port bound to `${BROWSER_BIND_ADDR}` (no default) and the resource
  limits from the epic: 512 MiB / 1 GiB memory+swap, `oom_score_adj 800`,
  halved CPU weight, shm 1 GiB -> 128 MiB.
- API stack drops the service, its `depends_on` and the `browser` network.
- `bookmark-api` gains the `default` network. Dropping `browser` had left it on
  `db` alone, which is `internal: true` — no published port and, worse, no
  egress for the poller at all. Caught by actually bringing the stack up.
- ADR-0006 for the topology; `DEPLOY.md` §7 for first-time setup of the browser
  machine; `REDEPLOY.md` §8 for its independent update cadence; architecture
  diagrams, config tables and troubleshooting rows across README/AGENTS/env.

### Verified locally

- Browser unit builds and runs: Chrome 151, UA carries no `HeadlessChrome`,
  all limits applied as declared.
- **Live smoke passes through the new unit**: `TestSmokeKaganeImage` fetched
  56710 bytes of `image/webp`, `TestSmokeKaganeGet` got a 200 with a real
  chapter list. The challenge cleared under the reduced 128 MiB shm.
- Bind isolation proven: refused on the host's non-loopback address, accepted
  on the configured one.
- 321 MiB peak of the 512 MiB cap after a full solve; 0 restarts, no OOM kill.
- API stack comes up clean, `/healthz` 200; egress confirmed present on
  `default` and absent on `db`.
- `go test ./...`, `go vet`, `gofmt` clean.

### Left to the operator

Provisioning the home machine, the Tailscale ACL, setting `BROWSER_WS_URL` in
production, and observing acceptance criteria 5-7 (covers with the machine off,
several days of zero OOM/restarts, VPS memory improvement). `DEPLOY.md` §7 now
carries the before/after `free -m` reading those need.

Reviewed-on: #52
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/internal/latest/smoke_image_test.go

@@ -9,11 +9,14 @@ import (
 )
 
 // TestSmokeKaganeImage is the live proof that the cover proxy's fetch actually
-// clears Cloudflare and returns image bytes. It needs a real headless Chrome
+// clears Cloudflare and returns image bytes. It needs the real browser unit
 // with outbound network, so it runs only when SMOKE_BROWSER_WS_URL is set:
 //
-//	docker run --rm --shm-size=1gb -p 19222:9222 chromedp/headless-shell:stable
-//	SMOKE_BROWSER_WS_URL=ws://127.0.0.1:19222 go test -run TestSmokeKaganeImage ./internal/latest
+//	cd chrome && BROWSER_BIND_ADDR=127.0.0.1 docker compose up -d --build
+//	SMOKE_BROWSER_WS_URL=ws://127.0.0.1:9222 go test -run TestSmokeKaganeImage ./internal/latest
+//
+// Not chromedp/headless-shell: its challenge never clears (see chrome/Dockerfile),
+// so a red run there proves nothing about kagane.
 func TestSmokeKaganeImage(t *testing.T) {
 	ws := os.Getenv("SMOKE_BROWSER_WS_URL")
 	if ws == "" {