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/main_test.go

@@ -17,19 +17,20 @@ import (
 func TestLoadLatestPollDefaults(t *testing.T) {
 	for _, k := range []string{
 		"LATEST_CHAPTER_POLL_ENABLED", "LATEST_CHAPTER_POLL_COOLDOWN",
-		"LATEST_CHAPTER_POLL_INTERVAL", "LATEST_CHAPTER_POLL_STAGGER",
-		"LATEST_CHAPTER_POLL_BATCH",
+		"LATEST_CHAPTER_POLL_BROWSER_COOLDOWN", "LATEST_CHAPTER_POLL_INTERVAL",
+		"LATEST_CHAPTER_POLL_STAGGER", "LATEST_CHAPTER_POLL_BATCH",
 	} {
 		t.Setenv(k, "")
 	}
 
 	got := loadLatestPoll()
 	want := LatestPoll{
-		Enabled:  true,
-		Cooldown: time.Hour,
-		Interval: 10 * time.Minute,
-		Stagger:  20 * time.Second,
-		Batch:    14,
+		Enabled:         true,
+		Cooldown:        time.Hour,
+		BrowserCooldown: 6 * time.Hour,
+		Interval:        10 * time.Minute,
+		Stagger:         20 * time.Second,
+		Batch:           14,
 	}
 	if got != want {
 		t.Fatalf("loadLatestPoll() = %+v, want %+v", got, want)
@@ -74,6 +75,30 @@ func TestLoadLatestPollClampsAndFallsBack(t *testing.T) {
 			wantFrom: func(p LatestPoll) any { return p.Cooldown },
 			want:     15 * time.Minute,
 		},
+		{
+			name:     "browser cooldown below the floor is clamped up",
+			env:      map[string]string{"LATEST_CHAPTER_POLL_BROWSER_COOLDOWN": "1m"},
+			wantFrom: func(p LatestPoll) any { return p.BrowserCooldown },
+			want:     15 * time.Minute,
+		},
+		{
+			name:     "browser cooldown at the floor is kept",
+			env:      map[string]string{"LATEST_CHAPTER_POLL_BROWSER_COOLDOWN": "15m"},
+			wantFrom: func(p LatestPoll) any { return p.BrowserCooldown },
+			want:     15 * time.Minute,
+		},
+		{
+			name:     "browser cooldown override is honoured",
+			env:      map[string]string{"LATEST_CHAPTER_POLL_BROWSER_COOLDOWN": "8h"},
+			wantFrom: func(p LatestPoll) any { return p.BrowserCooldown },
+			want:     8 * time.Hour,
+		},
+		{
+			name:     "browser cooldown unparseable value falls back",
+			env:      map[string]string{"LATEST_CHAPTER_POLL_BROWSER_COOLDOWN": "six hours"},
+			wantFrom: func(p LatestPoll) any { return p.BrowserCooldown },
+			want:     6 * time.Hour,
+		},
 		{
 			name:     "a valid override is honoured",
 			env:      map[string]string{"LATEST_CHAPTER_POLL_INTERVAL": "5m"},
@@ -123,6 +148,16 @@ func TestLoadLatestPollClampsAndFallsBack(t *testing.T) {
 	}
 }
 
+func TestNewLatestPollerWiresCooldowns(t *testing.T) {
+	p := newLatestPoller(nil, LatestPoll{
+		Cooldown:        time.Hour,
+		BrowserCooldown: 6 * time.Hour,
+	}, nil, nil)
+	if p.Cooldown != time.Hour || p.BrowserCooldown != 6*time.Hour {
+		t.Fatalf("poller cooldowns = %s/%s, want 1h/6h", p.Cooldown, p.BrowserCooldown)
+	}
+}
+
 func TestPutStatusValidation(t *testing.T) {
 	cases := []struct {
 		name   string