Add comix.to and kagane.to support (#13)

Tracks read progress on comix.to and kagane.to alongside asura and demonic, in both the userscript and the backend.

Implements `docs/superpowers/plans/2026-08-03-comix-kagane-support.md`.

## Userscript

- `comix` adapter — `/title/<id>-<slug>`; only the id prefix is identity (the slug follows the title). No `og:image`, so the cover is matched by `alt`.
- `kagane` adapter — reader URLs are uuids with no chapter number, so it comes out of `og:title`; anchor scanning is structurally impossible, replaced by `latestChapterFromApi` against kagane's same-origin JSON API.
- `seriesId` threaded through `latestChapterFromAnchors` so comix can scope its scan to its own series and a recommendation strip cannot win the maximum.
- `@match` for both hosts, panel chips, v1.6.0.

## Backend

- `latestChapterFrom` cases: comix parses the SSR JSON state blob (`latestChapterUrl`, scoped to the series id); kagane parses API JSON (`chapter_no`).
- Poller allowlist extended; `Poller.BrowserFetch` with `fetcherFor(site)` routes kagane to a browser fetcher. Nil means kagane is not polled at all — never a fallback to the TLS fetcher, which would only ever retrieve a challenge page.
- `BrowserFetcher`: chromedp against a `headless-shell` sidecar. kagane sits behind a Cloudflare JS challenge that no TLS fingerprint clears, and the request is made inside the page rather than by replaying `cf_clearance`.
- `BROWSER_WS_URL` wiring, sidecar in both compose files (no `ports:`, dedicated non-external network), Dockerfile on `golang:1.26-alpine` — chromedp requires go 1.26.
- Web UI `--comix` / `--kagane` tokens in both colour branches.

## Notes for review

- `series_url` is client-supplied and a headless browser is a strong SSRF primitive, so kagane's host is pinned twice: in `fetchableSeriesURL` and again in `kaganeAPIURL`.
- Three chained defects found during verification made the browser path dead under Compose (sidecar flag collision, Chrome's Host-header DNS-rebinding check, the wrong chromedp option). Fixed; the compose comments record the wrong configurations too, so they don't get "simplified" back.
- `ALLOWED_ORIGINS` now includes both new origins. Without it every write from comix/kagane silently fails CORS preflight, parks in the retry queue, and drops at the cap.

## Verification

221 backend tests, 32 userscript tests, static `CGO_ENABLED=0` build, both compose configs.

Two gaps, both real:

1. The userscript on live pages via Violentmonkey needs a human browser profile — not run. Check: comix series page (title/cover, no chapter), comix chapter page (records the number; an *older* chapter must not regress it), comix SPA navigation without reload, kagane series page (og:image cover), kagane reader (number from `og:title`), both chips opening the right sites.
2. The kagane browser path has not completed end-to-end anywhere. Dial/navigate/fetch is confirmed, but Cloudflare 403'd headless-shell's Chrome on every attempt from the dev sandbox, and comix's poll-through-Docker was blocked by that environment's TLS interception. Both environment-dependent rather than branch defects — the first real deploy is the actual verification.

Reviewed-on: #13
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

docker-compose.prod.yml

@@ -17,8 +17,19 @@ services:
   manga-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: ${BROWSER_WS_URL:-ws://172.28.0.10:9222}
+    depends_on:
+      - headless-shell
+    # `networks:` here replaces the base file's list entirely, so both must be
+    # named: `proxy` for Traefik routing, `browser` (defined in the base file)
+    # to keep reaching headless-shell without putting it on `proxy` too.
     networks:
       - proxy
+      - browser
     labels:
       - "traefik.enable=true"
       - "traefik.docker.network=${PROXY_NETWORK:-proxy}"
@@ -36,6 +47,11 @@ services:
       - "traefik.http.routers.mangaweb.tls.certresolver=${TRAEFIK_CERTRESOLVER:-le}"
       - "traefik.http.routers.mangaweb.service=mangabm"
 
+  # 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.
+
 networks:
   proxy:
     external: true