feat(visual-review): worker scaffold + webhook/queue + D1/R2

migrations/0037_visual_review_targets.sql

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+-- #578 (epic #577, roadmap #525): owner-led visual-change PR auto-review scaffold.
+-- Mirrors the submission-gate pattern (webhook -> queue -> D1 state) for a SEPARATE concern: visual
+-- review of UI-changing PRs. This migration lands only the state plumbing; later child issues add
+-- detection (#579), render/capture (#580/#581), diff (#582), R2 image hosting + managed comment (#583).
+--
+-- `visual_review_targets` tracks one review target per (repo, PR, head SHA): a fresh push (new head_sha)
+-- becomes a new target so each commit is reviewed independently and idempotently. Lifecycle states:
+--   queued -> capturing -> posted        (happy path; "posted" arrives once #583 ships the comment)
+--   queued -> capturing -> failed        (terminal failure after the queue/DLQ exhausts retries)
+-- `attempts` / `last_error` make a persistently-failing target observable instead of silently dropped.
+CREATE TABLE IF NOT EXISTS visual_review_targets (
+  id TEXT PRIMARY KEY,
+  repo_full_name TEXT NOT NULL,
+  pull_number INTEGER NOT NULL,
+  head_sha TEXT NOT NULL,
+  base_sha TEXT,
+  installation_id INTEGER,
+  status TEXT NOT NULL DEFAULT 'queued',
+  attempts INTEGER NOT NULL DEFAULT 0,
+  last_error TEXT,
+  delivery_id TEXT,
+  created_at TEXT NOT NULL,
+  updated_at TEXT NOT NULL
+);
+
+-- Idempotency: re-delivery of the same PR/head SHA webhook upserts the existing target rather than
+-- spawning duplicates (the submission-gate house pattern for webhook redelivery).
+CREATE UNIQUE INDEX IF NOT EXISTS visual_review_targets_repo_pr_head_unique
+  ON visual_review_targets (repo_full_name, pull_number, head_sha);
+CREATE INDEX IF NOT EXISTS visual_review_targets_repo_status_idx
+  ON visual_review_targets (repo_full_name, status);
+CREATE INDEX IF NOT EXISTS visual_review_targets_status_updated_idx
+  ON visual_review_targets (status, updated_at);
+
+-- Per-repository enablement. Visual review is OPT-IN (default disabled): a repo only gets visual-change
+-- PRs enqueued once an owner/operator flips `enabled`. Absence of a row == disabled.
+CREATE TABLE IF NOT EXISTS visual_review_settings (
+  repo_full_name TEXT PRIMARY KEY,
+  enabled INTEGER NOT NULL DEFAULT 0,
+  created_at TEXT NOT NULL,
+  updated_at TEXT NOT NULL
+);