Add pipeline studio web UI (demucs -> AuK -> MOSS-TTS)

Gradio app that orchestrates the three existing isolated venvs via
subprocess (their dependencies conflict too much to merge into one
environment - MOSS-TTS needs transformers 5.0.0, AuK needs 4.57.6).

Lets you strip a voice, iteratively edit it (emotion/pitch/speed/custom
instruction) with preview and undo/reset after each step, then generate
new speech in the resulting voice - addressing the trial-and-error
workflow the pitch-overshoot investigation needed.

Also extracts tts/generate.py as a reusable, general-purpose MOSS-TTS
cloning CLI (text/reference/language/output args) rather than one-off
personal scripts.

Verified all three stages end to end via direct function calls before
wiring up the UI.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Y1oQ4h4WE4R9KRvMQLr5Ey

webui/README.md

@@ -0,0 +1,56 @@
+# webui
+
+A pipeline studio: strip a voice with **demucs**, edit it with **AuK** (emotion, pitch, speed,
+free-form instructions — with preview and undo/reset), then generate new speech in that voice
+with **MOSS-TTS**.
+
+## Why this exists
+
+Manually chaining demucs → AuK → MOSS-TTS across three separate venvs (via SSH one-off scripts)
+worked but made it slow to experiment — especially with AuK's pitch/emotion controls, where you
+really want to preview a result and back out if it's wrong (see the pitch-overshoot findings in
+`auk/README.md`) rather than committing to a specific edit blind.
+
+## Architecture
+
+A single lightweight Gradio app (`app.py`) that **shells out** to each tool's own isolated venv
+via subprocess, rather than merging their dependencies into one environment — MOSS-TTS pins
+`transformers==5.0.0`, AuK needs `4.57.6`; they can't coexist in one venv. This app's own venv
+only needs `gradio` itself.
+
+Each stage's output becomes the next stage's input by filesystem path; a `history` list backs
+the undo/reset buttons in the voice editor.
+
+## Setup
+
+```bash
+python3 -m venv .venv && . .venv/bin/activate
+pip install gradio
+```
+
+Assumes `demucs/`, `auk/`, and `tts/` (siblings in this repo) already have their own working
+venvs and, for `auk/`, downloaded model weights — see their READMEs.
+
+## Running
+
+```bash
+python3 app.py
+```
+
+Serves on `0.0.0.0:7860`. The app automatically calls ComfyUI's `/free` endpoint (if its queue
+is empty) before each heavy step, to reduce GPU contention on a shared box — see the DNS/routing
+and GPU-contention notes earlier in this project's history for why that matters here.
+
+## Known limitations (inherited from the underlying tools)
+
+- **AuK's real usable clip length is ~25-30s**, not several minutes — feeding it a long recording
+  will OOM even with `--cpu_offload` (see `auk/README.md`). This app doesn't chunk long inputs
+  automatically; keep reference clips short.
+- **AuK's pitch-editing instruction-following is unreliable** — even "1 semitone" has measured
+  out at ~4+ semitones of actual shift in testing. The UI surfaces a warning; preview before
+  trusting a pitch edit.
+- Each stage is a blocking subprocess call (model load + inference each time) — the UI will
+  appear to hang for anywhere from ~10s to a couple of minutes per click, with no progress bar
+  yet.
+- No queueing/locking across browser tabs or concurrent users — this is a single-GPU box, so two
+  simultaneous generations will contend for VRAM.