docs: add playback selector plan

docs/plans/2026-02-01-use-playback-selector.md

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+# Use Playback Selector Implementation Plan
+
+> **For Claude:** REQUIRED SUB-SKILL: Use superpowers:executing-plans to implement this plan task-by-task.
+
+**Goal:** Add an asset variants endpoint and wire MediaPanel to use `pickVideoPlaybackVariant` for derived MP4 selection with a safe fallback.
+
+**Architecture:** Introduce a minimal `/api/assets/:id/variants` route that returns `{ kind, size, key }` from `asset_variants`. MediaPanel fetches variants on-demand for videos, uses `pickVideoPlaybackVariant` to decide whether to request `video_mp4` (size 720), and falls back to original if the derived URL fails.
+
+**Tech Stack:** Next.js App Router API routes, Postgres via `@tline/db`, Bun test runner.
+
+### Task 1: Add variants API route
+
+**Files:**
+- Create: `apps/web/app/api/assets/[id]/variants/route.ts`
+- Test: `apps/web/src/__tests__/variants-route.test.ts`
+
+**Step 1: Write the failing test**
+
+Create `apps/web/src/__tests__/variants-route.test.ts`:
+
+```ts
+import { test, expect } from "bun:test";
+
+test("variants route returns only kind/size/key fields", async () => {
+  const { shapeVariants } = await import("../../app/api/assets/[id]/variants/shape");
+  const rows = [
+    { kind: "video_mp4", size: 720, key: "derived/video/a.mp4", mime_type: "video/mp4" },
+  ];
+  expect(shapeVariants(rows)).toEqual([{ kind: "video_mp4", size: 720, key: "derived/video/a.mp4" }]);
+});
+```
+
+**Step 2: Run test to verify it fails**
+
+Run: `bun test apps/web/src/__tests__/variants-route.test.ts`
+Expected: FAIL (missing module or function)
+
+**Step 3: Write minimal implementation**
+
+- Create `apps/web/app/api/assets/[id]/variants/shape.ts` with `shapeVariants(rows)` that returns `{ kind, size, key }` only.
+- Create `apps/web/app/api/assets/[id]/variants/route.ts`:
+  - Validate `id` with `z.string().uuid()`
+  - Query `asset_variants` by `asset_id`
+  - Return JSON array of `shapeVariants(rows)`
+
+**Step 4: Run test to verify it passes**
+
+Run: `bun test apps/web/src/__tests__/variants-route.test.ts`
+Expected: PASS
+
+**Step 5: Commit**
+
+```bash
+git add apps/web/app/api/assets/[id]/variants/route.ts apps/web/app/api/assets/[id]/variants/shape.ts \
+  apps/web/src/__tests__/variants-route.test.ts
+git commit -m "feat: add asset variants endpoint"
+```
+
+### Task 2: Use playback selector in MediaPanel
+
+**Files:**
+- Modify: `apps/web/app/components/MediaPanel.tsx`
+- Modify: `apps/web/app/lib/playback.ts`
+- Test: `apps/web/src/__tests__/prefer-derived.test.ts`
+
+**Step 1: Write the failing test**
+
+Add to `apps/web/src/__tests__/prefer-derived.test.ts`:
+
+```ts
+import { test, expect } from "bun:test";
+import { pickVideoPlaybackVariant } from "../../app/lib/playback";
+
+test("pickVideoPlaybackVariant returns null when no variants", () => {
+  expect(
+    pickVideoPlaybackVariant({
+      originalMimeType: "video/x-matroska",
+      variants: [],
+    }),
+  ).toBeNull();
+});
+```
+
+**Step 2: Run test to verify it fails**
+
+Run: `bun test apps/web/src/__tests__/prefer-derived.test.ts`
+Expected: FAIL (function does not handle empty variants)
+
+**Step 3: Write minimal implementation**
+
+- Update `pickVideoPlaybackVariant` to return `null` when no `video_mp4` variants exist.
+- Update `MediaPanel` video URL loader to:
+  1) Fetch `/api/assets/:id/variants`
+  2) Call `pickVideoPlaybackVariant`
+  3) If variant found → request `kind=video_mp4&size=720`
+  4) If not found or fetch fails → request `variant=original`
+
+**Step 4: Run test to verify it passes**
+
+Run: `bun test apps/web/src/__tests__/prefer-derived.test.ts`
+Expected: PASS
+
+**Step 5: Commit**
+
+```bash
+git add apps/web/app/components/MediaPanel.tsx apps/web/app/lib/playback.ts \
+  apps/web/src/__tests__/prefer-derived.test.ts
+git commit -m "fix: use playback selector in MediaPanel"
+```