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.

chrome/.env.example

@@ -0,0 +1,28 @@
+# Copy to chrome/.env on the home machine. Never commit the real .env.
+#
+# This file configures the browser unit only. It is separate from the API
+# stack's ../.env on purpose: the two run on different machines.
+
+# The address the CDP port is published on — required, no default.
+#
+# Use this machine's **tailnet IP**, e.g. 100.x.y.z (`tailscale ip -4`). Not
+# 0.0.0.0, not the LAN address: CDP has no authentication of its own, so
+# anything that can reach this port has full control of the browser and a
+# foothold on this host. Tailscale device identity plus an ACL is the access
+# control; the bind address is what enforces it.
+#
+# For a throwaway local test, 127.0.0.1 is fine — but then only this machine
+# can reach it, so the API must run here too.
+BROWSER_BIND_ADDR=100.x.y.z
+
+# Clock zone the browser reports. A UTC clock is itself the bot signal —
+# Cloudflare treats it as the datacenter default — and kagane's challenge then
+# never clears. Measured 2026-08-08, identical container, one Indonesian egress
+# IP: UTC never cleared in 60s (twice); Asia/Jakarta and America/New_York both
+# cleared in 4s. So any real zone works; it does not have to match the IP's
+# country, it just must not be UTC.
+#
+# Unset falls back to the host's /etc/timezone, which is a real zone whenever
+# the host clock is set to local time. Set this when the host runs UTC — a UTC
+# server is exactly the case that fails.
+# BROWSER_TZ=Asia/Jakarta

chrome/docker-compose.yml

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+# The browser, as its own deployable unit.
+#
+# This does NOT run beside the API. It runs on the home machine, reached from
+# the VPS over the tailnet, and is updated without touching the API stack:
+#
+#   cd chrome && docker compose up -d --build
+#
+# Set BROWSER_BIND_ADDR in chrome/.env to this machine's tailnet IP. See
+# ../DEPLOY.md §7 for the full first-time procedure and ../docs/adr/
+# 0006-browser-on-the-home-machine.md for why the browser lives here at all.
+name: bookmark-browser
+
+services:
+  browser:
+    build: .
+    image: bookmarkmanager-chrome:latest
+    container_name: bookmark-browser
+    restart: unless-stopped
+    environment:
+      # A UTC clock is itself the bot signal: Cloudflare treats it as the
+      # datacenter default, and kagane's challenge then never clears. Measured
+      # 2026-08-08, identical container, one Indonesian egress IP: UTC never
+      # cleared in 60s (twice); Asia/Jakarta and America/New_York both cleared
+      # in 4s. So any real zone works and it need not match the IP's country —
+      # only UTC fails. Unset falls back to the host's /etc/timezone below,
+      # which is a real zone whenever the host clock is set to local time; set
+      # BROWSER_TZ when the host runs UTC.
+      TZ: ${BROWSER_TZ:-}
+    volumes:
+      # The zone *name*, which is what Chrome's ICU needs — see entrypoint.sh.
+      # Absent on a non-Debian host, which the entrypoint handles by falling back to UTC.
+      - /etc/timezone:/etc/timezone:ro
+      # Cloudflare clearance must survive Chrome reaping and image recreation.
+      - chrome-profile:/home/chrome/profile
+    # Bound to the tailnet address only, never 0.0.0.0. CDP authenticates
+    # nothing: whatever reaches this port drives the browser and, through it,
+    # this host. On the VPS the safety was Docker network membership; here the
+    # machine has a real LAN, so the bind address *is* the access control,
+    # backed by Tailscale device identity. No default — an unset variable must
+    # fail the deploy rather than silently publish CDP to the LAN.
+    ports:
+      - "${BROWSER_BIND_ADDR:?set BROWSER_BIND_ADDR to this machine's tailnet IP}:9222:9222"
+    # Reaps zombie renderer processes, which otherwise accumulate for the
+    # container's lifetime.
+    init: true
+    # Chrome allocates shared memory per tab and dies on Docker's 64MB default.
+    # 128MB against a measured 19MB peak: the old 1GB reservation was sized by
+    # superstition, and this box has 1.8GB total.
+    shm_size: '128mb'
+    # The browser is the newcomer on a machine where a Gitea runner already
+    # holds ~1.2GiB of 1.8GiB. Load-bearing, not decorative: untuned Chrome
+    # peaked at 645MiB cgroup, which is more than is free here.
+    #
+    # memswap_limit is memory+swap combined, so this allows 512MiB of swap —
+    # Chrome reclaims its own cold pages onto this box's 5.9GiB of SATA swap
+    # instead of taking resident memory from the runner.
+    mem_limit: 512m
+    memswap_limit: 1g
+    # If the box does run out, the kernel takes the browser and never CI.
+    oom_score_adj: 800
+    # A challenge solve yields to a running build. Cold start degrades to ~3s
+    # at half a CPU, immaterial against a 45-second challenge budget.
+    cpu_shares: 512
+
+volumes:
+  chrome-profile: