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>

.env.example

@@ -67,13 +67,15 @@ DISCORD_REDIRECT_URI=
 # Set to 0 to turn it off entirely.
 # LATEST_CHAPTER_POLL_ENABLED=1
 #
-# Two independent clocks. COOLDOWN is how long one series rests between checks;
-# INTERVAL is how often the poller wakes up and looks for series past that
-# cooldown. Shortening INTERVAL cannot shorten a COOLDOWN.
-# LATEST_CHAPTER_POLL_COOLDOWN=1h     # per series, floor 15m
-# LATEST_CHAPTER_POLL_INTERVAL=10m    # how often to wake
-# LATEST_CHAPTER_POLL_BATCH=14        # series per wake
-# LATEST_CHAPTER_POLL_STAGGER=20s     # delay between fetches in a batch
+# Two independent clocks. COOLDOWN is how long a plain-TLS series rests between
+# checks; BROWSER_COOLDOWN is the longer rest for kagane and novelfull. INTERVAL
+# is how often the poller wakes up and looks for series past their cooldowns.
+# Shortening INTERVAL cannot shorten either cooldown.
+LATEST_CHAPTER_POLL_COOLDOWN=1h         # plain-TLS per series, floor 15m
+LATEST_CHAPTER_POLL_BROWSER_COOLDOWN=6h # browser-backed per series, floor 15m
+LATEST_CHAPTER_POLL_INTERVAL=10m        # how often to wake
+LATEST_CHAPTER_POLL_BATCH=14            # series per wake
+LATEST_CHAPTER_POLL_STAGGER=20s         # delay between fetches in a batch
 #
 # Uses a ticker, not an immediate first run: the first poll happens one
 # INTERVAL after startup, not at startup. A container restarting more often