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>

.env.example

@@ -5,8 +5,8 @@
 API_TOKEN=changeme-generate-a-long-random-token
 
 # Comma-separated origins allowed to call the API (CORS). Both Asura domains
-# plus Demonic. Add/remove as the sites' hostnames change.
-ALLOWED_ORIGINS=https://asuracomic.net,https://asurascans.com,https://demonicscans.org
+# plus Demonic, Comix, and Kagane. Add/remove as the sites' hostnames change.
+ALLOWED_ORIGINS=https://asuracomic.net,https://asurascans.com,https://demonicscans.org,https://comix.to,https://kagane.to
 
 # --- Prod override (Traefik) only ---
 # Subdomain Traefik routes to this service (required by the prod override).
@@ -52,3 +52,11 @@ WEB_PASSWORD=
 # BATCH x (COOLDOWN / INTERVAL) series hold the cooldown cadence — 84 with these
 # defaults. Beyond that the cadence stretches uniformly rather than breaking;
 # raise BATCH or lower INTERVAL. Keep BATCH x STAGGER under INTERVAL.
+
+# Headless-shell CDP endpoint for sites behind a JavaScript challenge (kagane).
+# Unset disables browser polling; those sites then rely on the userscript alone.
+# Leave commented — the compose files' own default (ws://172.28.0.10:9222) is
+# correct. Do NOT set this to the "headless-shell" DNS name: Chrome's DevTools
+# HTTP handler 500s any /json/version request whose Host header isn't an IP or
+# "localhost", which silently breaks every kagane poll.
+# BROWSER_WS_URL=ws://172.28.0.10:9222