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_test.go

@@ -291,7 +291,7 @@ func TestDueForLatestCheck(t *testing.T) {
 			s := newTestStore(t)
 			seedForCheck(t, s, "asura:x", tt.seriesURL, tt.checkedAt)
 
-			due, err := s.DueForLatestCheck(now-hour, 10)
+			due, err := s.DueForLatestCheck(now-hour, now-hour, nil, 10)
 			if err != nil {
 				t.Fatalf("DueForLatestCheck: %v", err)
 			}
@@ -309,7 +309,7 @@ func TestDueForLatestCheckOldestFirstAndLimited(t *testing.T) {
 	seedForCheck(t, s, "asura:b", "https://asurascans.com/comics/b", 200)
 	seedForCheck(t, s, "asura:a", "https://asurascans.com/comics/a", 100)
 
-	due, err := s.DueForLatestCheck(1000, 2)
+	due, err := s.DueForLatestCheck(1000, 1000, nil, 2)
 	if err != nil {
 		t.Fatalf("DueForLatestCheck: %v", err)
 	}
@@ -489,7 +489,7 @@ func TestDueForLatestCheckSkipsFinishedKeepsArchived(t *testing.T) {
 		}
 	}
 
-	due, err := store.DueForLatestCheck(time.Now().UnixMilli(), 10)
+	due, err := store.DueForLatestCheck(time.Now().UnixMilli(), time.Now().UnixMilli(), nil, 10)
 	if err != nil {
 		t.Fatalf("DueForLatestCheck: %v", err)
 	}
@@ -844,7 +844,7 @@ func TestDueForLatestCheckOrdersByReaderCountThenAge(t *testing.T) {
 	seedSecondReader(t, s, "asura:pop:2", "asura", "pop", 1001)
 	seedForCheck(t, s, "asura:solo", "https://asurascans.com/comics/solo", 100)
 
-	due, err := s.DueForLatestCheck(1000, 10)
+	due, err := s.DueForLatestCheck(1000, 1000, nil, 10)
 	if err != nil {
 		t.Fatalf("DueForLatestCheck: %v", err)
 	}
@@ -870,7 +870,7 @@ func TestDueForLatestCheckExcludesOrphanSeries(t *testing.T) {
 		t.Fatalf("seed orphan series: %v", err)
 	}
 
-	due, err := s.DueForLatestCheck(1000, 10)
+	due, err := s.DueForLatestCheck(1000, 1000, nil, 10)
 	if err != nil {
 		t.Fatalf("DueForLatestCheck: %v", err)
 	}
@@ -1207,7 +1207,7 @@ func TestTwoReadersShareOneSeriesWithIndependentProgress(t *testing.T) {
 		t.Fatalf("series rows = %d, want 1 shared row for two bookmarks", series)
 	}
 
-	due, err := s.DueForLatestCheck(time.Now().UnixMilli(), 10)
+	due, err := s.DueForLatestCheck(time.Now().UnixMilli(), time.Now().UnixMilli(), nil, 10)
 	if err != nil {
 		t.Fatalf("DueForLatestCheck: %v", err)
 	}
@@ -1223,7 +1223,7 @@ func TestTwoReadersShareOneSeriesWithIndependentProgress(t *testing.T) {
 	if b, ok, err := s.Get(s.OwnerID(), "asura:solo"); err != nil || !ok || b.LastChapterNum != 200 {
 		t.Fatalf("owner's bookmark after the other's delete = %+v ok=%v err=%v, want it intact", b, ok, err)
 	}
-	due, err = s.DueForLatestCheck(time.Now().UnixMilli(), 10)
+	due, err = s.DueForLatestCheck(time.Now().UnixMilli(), time.Now().UnixMilli(), nil, 10)
 	if err != nil {
 		t.Fatalf("DueForLatestCheck after delete: %v", err)
 	}