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/internal/store/store.go

@@ -12,6 +12,7 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/jackc/pgx/v5/pgtype"
 	_ "github.com/jackc/pgx/v5/stdlib"
 )
 
@@ -689,16 +690,17 @@ func (s *Store) Delete(readerID int64, key string) error {
 }
 
 // DueForLatestCheck returns series whose server-side latest-chapter check has
-// aged past cutoffMs, ordered by how many bookmarks reference them (descending)
-// then least-recently-checked first, at most limit of them.
+// aged past the appropriate cutoff, ordered by how many bookmarks reference
+// them (descending) then least-recently-checked first, at most limit of them.
+// Browser-backed sites use browserCutoffMs; every other site uses cutoffMs.
 //
 // The reader_count ordering is the point of the split (ADR-0003): a series
 // shared by several readers is fetched once per due cycle, and the popular
 // ones stay freshest while the long tail absorbs any shortfall. Within one
-// reader count, oldest-first keeps the poll fair when the backlog outgrows its
+// reader count, oldest-first keeps the poll fair when the backlog outgrows
 // throughput: the most neglected series is always next, so a large collection
-// refreshes uniformly slower rather than leaving a tail that never refreshes at
-// all. The userscript sorts its own queue the same way (L453).
+// refreshes uniformly slower rather than leaving a tail that never refreshes
+// at all. The userscript sorts its own queue the same way (L453).
 //
 // Series with no series_url are skipped — there is nothing to fetch, which is
 // the same filter the userscript applies at L452. Series whose only bookmarks
@@ -706,17 +708,20 @@ func (s *Store) Delete(readerID int64, key string) error {
 // burns requests. Archived bookmarks still count — knowing what a shelved
 // series is up to is the whole reason for archiving instead of deleting.
 // A series with no bookmarks at all never appears: the join excludes it.
-func (s *Store) DueForLatestCheck(cutoffMs int64, limit int) ([]Series, error) {
+func (s *Store) DueForLatestCheck(cutoffMs, browserCutoffMs int64, browserSites []string, limit int) ([]Series, error) {
 	rows, err := s.db.Query(`SELECT `+seriesColumns+`, COUNT(*) AS reader_count
 		FROM series s
 		JOIN bookmarks b ON b.site = s.site AND b.series_id = s.series_id
 		WHERE s.series_url <> ''
-		  AND s.latest_checked_at <= $1
+		  AND s.latest_checked_at <= CASE
+			WHEN s.site = ANY($3::text[]) THEN $2::bigint
+			ELSE $1::bigint
+		  END
 		GROUP BY s.site, s.series_id, s.title, s.series_url, s.cover,
 		         s.kind, s.latest_chapter, s.latest_chapter_num, s.latest_checked_at
 		HAVING COUNT(*) FILTER (WHERE b.status <> 'finished') > 0
 		ORDER BY reader_count DESC, s.latest_checked_at ASC
-		LIMIT $2`, cutoffMs, limit)
+		LIMIT $4`, cutoffMs, browserCutoffMs, pgtype.FlatArray[string](browserSites), limit)
 	if err != nil {
 		return nil, fmt.Errorf("query due series: %w", err)
 	}