feat: initial scaffold for ClickTrack monorepo

Full self-hosted click track generator for cover bands.

Core technical pieces implemented:
- CTP (Click Track Protocol) TypeScript schema, Zod validator, and WAV
  renderer (44.1 kHz, 16-bit PCM, accented downbeats, ramp sections)
- MusicBrainz API client with 1 req/s rate limiting
- PostgreSQL schema (songs, tempo_maps, registry_sync_log) with triggers
- Git registry sync logic (clone/pull → validate CTP → upsert DB)
- Next.js 14 App Router: search page, track page, API routes
  (/api/songs, /api/tracks, /api/generate)
- UI components: SearchBar, SongResult, TempoMapEditor, ClickTrackPlayer
  (Web Audio API in-browser playback + WAV download)
- Docker Compose stack: app + postgres + redis + nginx + registry-sync
- Multi-stage Dockerfile with standalone Next.js output
- .env.example documenting all configuration variables
- README with setup instructions, CTP format spec, and API reference

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,44 @@
+# ─────────────────────────────────────────────────────────────────────────────
+# ClickTrack — environment configuration
+# Copy this file to .env and fill in your values.
+# ─────────────────────────────────────────────────────────────────────────────
+
+# ── Database ─────────────────────────────────────────────────────────────────
+# PostgreSQL connection string.
+# When using docker compose the default works out of the box.
+DATABASE_URL=postgres://clicktrack:clicktrack@localhost:5432/clicktrack
+
+# Password used by the postgres service in docker-compose.yml.
+# Change this before deploying to production.
+POSTGRES_PASSWORD=clicktrack
+
+# ── Redis ────────────────────────────────────────────────────────────────────
+# Redis connection URL.
+REDIS_URL=redis://localhost:6379
+
+# ── Community registry ───────────────────────────────────────────────────────
+# Public GitHub repository containing community CTP files.
+# Example: https://github.com/your-org/clicktrack-registry
+# Leave blank to disable registry sync.
+REGISTRY_REPO=
+
+# Branch to pull from (default: main).
+REGISTRY_BRANCH=main
+
+# Interval in seconds between registry syncs (default: 3600 = 1 hour).
+REGISTRY_SYNC_INTERVAL=3600
+
+# ── App ──────────────────────────────────────────────────────────────────────
+# Display name shown in the UI and page title.
+NEXT_PUBLIC_APP_NAME=ClickTrack
+
+# ── MusicBrainz ──────────────────────────────────────────────────────────────
+# User-Agent string sent to MusicBrainz. Must identify your application and
+# provide a contact URL or email per their usage policy:
+# https://musicbrainz.org/doc/MusicBrainz_API/Rate_Limiting
+MUSICBRAINZ_USER_AGENT=ClickTrack/0.1 (https://your-instance-url)
+
+# ── Ports (docker-compose.yml) ───────────────────────────────────────────────
+# Host ports for the nginx reverse proxy.
+HTTP_PORT=80
+HTTPS_PORT=443