feat(latest): split browser poll cooldown (#48)

## Summary

Split latest-chapter polling cooldowns by fetch cost. Browser-backed kagane and novelfull series now rest longer without changing the cadence of plain-TLS sites.

## Behavior

- Plain-TLS series keep the 1h default cooldown.
- Browser-backed series use `LATEST_CHAPTER_POLL_BROWSER_COOLDOWN`, defaulting to 6h.
- Both cooldowns share the existing 15m minimum floor; invalid values retain the existing fallback behavior.
- The poller still selects both classes in one due query per cycle.
- Existing ordering and exclusions remain unchanged: reader-count precedence, least-recently-checked ordering, finished exclusion, archived polling, and orphan exclusion.

## Implementation

- Added the browser cooldown to backend configuration and passed it through production poller construction.
- Added the browser-site list as the single routing source used for both due-query cutoff selection and fetcher choice.
- Kept all query values parameterized; the site list is passed as a bound PostgreSQL array parameter.
- Updated startup logging to report interval, plain cooldown, browser cooldown, batch, and stagger.
- Documented the variable, default, and floor in `README.md`, `.env.example`, `backend/AGENTS.md`, and `docker-compose.yml`.

## Review findings addressed

The first review found that configuration parsing was correct but `startLatestPoller` did not pass `BrowserCooldown` into `latest.Poller`; every browser-backed row would therefore have been due immediately. Production construction now goes through `newLatestPoller`, with a regression test covering both cooldown fields.

The review also identified duplicated browser-site knowledge in fetch routing. `slices.Contains(browserBackedSites, site)` now reuses the same list already supplied to the store query.

## Verification

- Focused backend tests pass: `go test ./internal/latest ./internal/store .`.
- Full suite passes: `go test ./...`.
- `graphify update .` completed.
- Issue #42 was updated and closed.

Reviewed-on: #48
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

backend/AGENTS.md

@@ -118,8 +118,10 @@ Guidance for OpenCode (and Claude Code) working under `backend/`. See root `AGEN
   `_REDIRECT_URI` (required; Discord OAuth for the browser UI),
   `DISCORD_REQUIRED_ROLE` (optional role gate, empty by default),
   `DISCORD_API_BASE` (default `https://discord.com/api/v10`),
-  `LATEST_CHAPTER_POLL_ENABLED`/`_COOLDOWN`/`_INTERVAL`/`_BATCH`/`_STAGGER`
-  (background latest-chapter poller; defaults on, `1h`/`10m`/`14`/`20s`).
+  `LATEST_CHAPTER_POLL_ENABLED`/`_COOLDOWN`/`_BROWSER_COOLDOWN`/`_INTERVAL`/
+  `_BATCH`/`_STAGGER` (background latest-chapter poller; defaults on,
+  `1h` plain-TLS cooldown, `6h` browser cooldown, `10m`/`14`/`20s`; both
+  cooldowns have a `15m` floor).
   `USERSCRIPT_PATH` and `NOVEL_USERSCRIPT_PATH` (files served at
   `/u/{token}/manga-bookmark.user.js` and `/u/{token}/novel-bookmark.user.js`,
   defaults `/userscript/manga-bookmark.user.js` and