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/latest/poller.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"log"
 	"net/url"
+	"slices"
 	"time"
 
 	"bookmarkmanager/backend/internal/store"
@@ -23,34 +24,36 @@ type Fetcher interface {
 // Two clocks, deliberately independent:
 //
 //   - Interval is how often this goroutine wakes up and looks.
-//   - Cooldown is how long one series rests since its own last check.
+//   - Cooldowns are how long a series rests since its own last check. Browser-
+//     backed sites use the longer BrowserCooldown.
 //
-// Only the cooldown is per series, and it is enforced by the WHERE clause in
-// DueForLatestCheck rather than by any timer. Shortening Interval therefore
-// cannot shorten anyone's cooldown; it only makes the poller wake up and find
-// nothing due more often.
+// Cooldowns are enforced by the WHERE clause in DueForLatestCheck rather than
+// by any timer. Shortening Interval therefore cannot shorten anyone's cooldown;
+// it only makes the poller wake up and find nothing due more often.
 type Poller struct {
 	Store *store.Store
 	Fetch Fetcher
 	// BrowserFetch handles sites behind a JavaScript challenge that Fetch
 	// cannot clear. Nil disables those sites entirely rather than falling back
 	// to Fetch, which would only ever retrieve a challenge page.
-	BrowserFetch Fetcher
-	Now          func() time.Time // injected so tests can freeze it
-	Cooldown     time.Duration
-	Interval     time.Duration
-	Stagger      time.Duration
-	Batch        int
+	BrowserFetch    Fetcher
+	Now             func() time.Time // injected so tests can freeze it
+	Cooldown        time.Duration
+	BrowserCooldown time.Duration
+	Interval        time.Duration
+	Stagger         time.Duration
+	Batch           int
 }
 
+var browserBackedSites = []string{"kagane", "novelfull"}
+
 // fetcherFor returns the fetcher a site needs, or nil when the site cannot be
 // fetched at all right now. kagane and novelfull both sit behind a Cloudflare
 // JavaScript challenge that no TLS fingerprint clears — kagane verified
 // 2026-08-03, novelfull verified 2026-08-05, both against the same Chrome_133
 // profile TLSFetcher uses — so they are browser-only or nothing.
 func (p *Poller) fetcherFor(site string) Fetcher {
-	switch site {
-	case "kagane", "novelfull":
+	if slices.Contains(browserBackedSites, site) {
 		return p.BrowserFetch
 	}
 	return p.Fetch
@@ -63,8 +66,8 @@ func (p *Poller) fetcherFor(site string) Fetcher {
 // failure mode for a misconfigured batch x stagger: a slower cadence, never
 // concurrent fetch storms.
 func (p *Poller) Run(ctx context.Context) {
-	log.Printf("latest-chapter poller: interval=%s cooldown=%s batch=%d stagger=%s",
-		p.Interval, p.Cooldown, p.Batch, p.Stagger)
+	log.Printf("latest-chapter poller: interval=%s cooldown=%s browser-cooldown=%s batch=%d stagger=%s",
+		p.Interval, p.Cooldown, p.BrowserCooldown, p.Batch, p.Stagger)
 	t := time.NewTicker(p.Interval)
 	defer t.Stop()
 	for {
@@ -80,8 +83,10 @@ func (p *Poller) Run(ctx context.Context) {
 
 // runOnce processes one batch of due series.
 func (p *Poller) runOnce(ctx context.Context) {
-	cutoff := p.Now().Add(-p.Cooldown).UnixMilli()
-	due, err := p.Store.DueForLatestCheck(cutoff, p.Batch)
+	now := p.Now()
+	cutoff := now.Add(-p.Cooldown).UnixMilli()
+	browserCutoff := now.Add(-p.BrowserCooldown).UnixMilli()
+	due, err := p.Store.DueForLatestCheck(cutoff, browserCutoff, browserBackedSites, p.Batch)
 	if err != nil {
 		log.Printf("latest poll: due query: %v", err)
 		return