feat: one Poll Lane per Site, replacing the shared pace (#100) (#106)

Closes #100.

Each Site runs its own Poll Lane: an independent goroutine with its own rest
and pace from the registry (`backend/internal/latest/sites.go`), replacing the
shared cooldown/interval/stagger/batch configuration. Rest (1h, all six Sites
including the browser trio) is enforced by the due query's WHERE clause; the
Lane sleeps its effective gap between fetches — the registry 10s, or
rest/eligible when a Site holds enough Series, floored at 1s with a
Site-naming warning when the floor engages.

Lane-local failure handling:
- Two challenge-held results stop that Site's Lane for 15m; the probes keep
  their stamp, untried Series stay due.
- A lost browser sets a shared Poller flag: the other browser Lanes skip
  their passes for the same 15m (no stamp-per-pass-per-Lane on a dead tab),
  then decay and probe again.
- Browser wake gate preserved (5 due, or one waiting 15m, ADR-0005); one tab
  shared by the three browser Sites; "browser lane behind by X" logged every
  pass.
- Cover work (healing a stored source URL and filling a blank from the series
  page) runs in the background so a slow CDN cannot consume a Lane's gap.

Removed: `LATEST_CHAPTER_POLL_{COOLDOWN,BROWSER_COOLDOWN,INTERVAL,BATCH,STAGGER}`
and the 6h browser rest. Only `LATEST_CHAPTER_POLL_ENABLED` remains; DEPLOY.md
documents the exact `.env` edit. ADR-0010 records the decisions.

Reviewed-on: #106
Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com>
Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>

graphify-out/.graphify_labels.json

@@ -30,7 +30,8 @@
   "28": "Allocation Patterns",
   "29": "Observability & Alerting",
   "30": "AGENTS.md",
-  "32": "Repo Hard Constraints",
+  "31": "Store",
+  "32": "Open",
   "33": "Go Testing Guide",
   "34": "Session Store",
   "35": "Web UI Filter Logic",
@@ -39,7 +40,7 @@
   "38": "novel-logic.test.js",
   "39": "UI Critique 2026-07-26A",
   "40": "UI Critique 2026-07-26B",
-  "41": "sessions_test.go",
+  "42": "pgtest.go",
   "43": "Issue Tracker & Triage",
   "44": "Ticket Workflow",
   "45": "Go Perf Alert Rules",
@@ -122,6 +123,7 @@
   "122": "Identity comes from Discord OAuth; we store no passwords and send no email",
   "123": "The wire format stays flat and deliberately does not mirror the schema",
   "124": "ADR-0005: On-demand browser sidecar",
+  "125": "sessions_test.go",
   "126": "Bookmark Manager",
   "127": "triage-labels.md",
   "128": "Cross-Ticket Contract",
@@ -149,7 +151,6 @@
   "150": "Reviewer Subagent (opencode)",
   "151": "Finding Severity Rubric",
   "152": "Spec Compliance Review",
-  "158": "T",
   "161": "Why Use samber/oops",
   "162": "singleflight Cache Stampede Prevention",
   "163": "Struct Field Alignment",