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

Each Site runs its own poll goroutine, paced by Rest and Gap from the
registry (sites.go) instead of the shared cooldown/interval/stagger/batch
config. Rest is enforced by the due query's WHERE clause; the Lane sleeps
its effective gap between fetches (hour/eligible, floored at 1s).

Lane-local failures: two refusals stop that Site for 15m, a lost browser
stops only the round's remaining browser Lanes, and cover heals moved to
background goroutines so a slow CDN cannot consume a Lane's gap. The five
LATEST_CHAPTER_POLL_* pace env vars are gone; only the kill switch
remains.

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",