docs(06-01): complete YouTube audio loading plan

- Create 06-01-SUMMARY.md with full execution details
- Update STATE.md: advance to plan 2, 90% progress, add decisions
- Update ROADMAP.md: phase 6 showing 1/3 plans complete

.planning/phases/06-ai-sync/06-01-SUMMARY.md

@@ -0,0 +1,91 @@
+---
+phase: 06-ai-sync
+plan: 01
+subsystem: audio-input
+tags: [youtube, yt-dlp, sse, streaming, frontend, transport-bar]
+dependency_graph:
+  requires: []
+  provides: [youtube-audio-download, sse-progress-stream, yt-dlp-dependency]
+  affects: [lightsync/api/audio.py, lightsync/frontend]
+tech_stack:
+  added: [yt-dlp>=2025.0.0, anthropic>=0.40.0]
+  patterns: [asyncio subprocess, SSE ReadableStream, terminal progress bar]
+key_files:
+  created: []
+  modified:
+    - pyproject.toml
+    - docker-compose.prod.yml
+    - lightsync/api/audio.py
+    - lightsync/frontend/index.html
+    - lightsync/frontend/app.js
+    - lightsync/frontend/style.css
+decisions:
+  - "sys.executable -m yt_dlp used to avoid PATH issues in Docker"
+  - "--newline flag required for line-by-line progress parsing from yt-dlp"
+  - "yt_%(id)s.%(ext)s output template ensures predictable filenames"
+  - "ANTHROPIC_API_KEY wired with empty default so container starts without it"
+  - "Beat analysis runs post-download (same as upload path) for identical audio pipeline entry"
+metrics:
+  duration: "~8 min"
+  completed: "2026-04-07"
+  tasks_completed: 3
+  files_changed: 6
+requirements_satisfied: [AUD-02]
+---
+
+# Phase 06 Plan 01: YouTube Audio Loading via yt-dlp Summary
+
+**One-liner:** YouTube URL download via yt-dlp subprocess with SSE progress streaming, terminal-style block progress bar, and automatic audio pipeline entry after download.
+
+## What Was Built
+
+Added full YouTube audio loading capability to the LightSync transport bar:
+
+1. **Dependencies (Task 1):** Added `yt-dlp>=2025.0.0` and `anthropic>=0.40.0` to `pyproject.toml`. Wired `ANTHROPIC_API_KEY` env var (empty default) in `docker-compose.prod.yml`. Both packages are importable inside the running container.
+
+2. **Backend endpoint (Task 2):** New `POST /api/audio/fetch` endpoint in `lightsync/api/audio.py` that:
+   - Accepts `{url: string}` JSON body
+   - Spawns yt-dlp subprocess via `sys.executable -m yt_dlp` (avoids PATH issues in Docker)
+   - Parses progress with `PROGRESS_RE = re.compile(r'\[download\]\s+([\d.]+)%')` using `--newline` flag
+   - Streams SSE progress events with `media_type="text/event-stream"`
+   - Runs beat analysis on completion and caches in `app.state.beats`
+   - Returns `{type: "done", path, filename}` event on success
+
+3. **Frontend UI (Task 3):** Added to transport bar in `index.html`:
+   - Text input `#yt-url` with placeholder "YOUTUBE URL"
+   - `#btn-fetch` button
+   - `#yt-progress` span for terminal-style progress display
+   
+   In `app.js`:
+   - `fetchWithProgress()` function using fetch ReadableStream reader for SSE
+   - `renderProgressBar()` rendering `DOWNLOADING ████████░░░░░░░░░░░░ 42%` style
+   - FETCH button handler with D-02 disable-during-download
+   - Auto-load into `#player` after download (identical to upload pipeline)
+   
+   In `style.css`: `.yt-url-input` and `.yt-progress` styles using existing CSS variables, flat/monospace/no border-radius.
+
+## Commits
+
+| Task | Commit | Description |
+|------|--------|-------------|
+| 1 | `165cdb6` | feat(06-01): add yt-dlp and anthropic dependencies |
+| 2 | `d58ea8c` | feat(06-01): add YouTube download endpoint with SSE progress streaming |
+| 3 | `dca1cb8` | feat(06-01): add YouTube URL input with terminal progress bar |
+
+## Deviations from Plan
+
+None - plan executed exactly as written.
+
+## Known Stubs
+
+None - YouTube download is fully wired end-to-end.
+
+## Self-Check: PASSED
+
+- `pyproject.toml` contains `yt-dlp>=2025.0.0` and `anthropic>=0.40.0`
+- `docker-compose.prod.yml` contains `ANTHROPIC_API_KEY`
+- `lightsync/api/audio.py` contains `async def fetch_youtube`, `PROGRESS_RE`, `media_type="text/event-stream"`, `class YouTubeFetchRequest`, `analyze_async(filename)`
+- `lightsync/frontend/index.html` contains `id="yt-url"`, `id="btn-fetch"`, `id="yt-progress"`
+- `lightsync/frontend/app.js` contains `fetchWithProgress`, `renderProgressBar`, `/api/audio/fetch`, `urlInput.disabled = true`, `audio.src`
+- `lightsync/frontend/style.css` contains `.yt-url-input`, `.yt-progress`
+- All 3 task commits exist: `165cdb6`, `d58ea8c`, `dca1cb8`