Add Lyra acquisition-engine design spec, README, gitignore

Initial design for slice 1 (acquisition engine): three-container
architecture (Next.js UI, Python worker, slskd), the six-stage
pipeline, source-adapter contract, data model, error handling, and
testing strategy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,39 @@
+# Dependencies
+node_modules/
+.pnp
+.pnp.js
+__pycache__/
+*.py[cod]
+.venv/
+venv/
+env/
+
+# Next.js
+.next/
+out/
+build/
+dist/
+
+# Env & secrets
+.env
+.env*.local
+*.local
+
+# Docker / runtime data
+music/
+data/
+postgres-data/
+slskd-data/
+
+# Logs
+*.log
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# OS / editor
+.DS_Store
+Thumbs.db
+.idea/
+.vscode/
+*.swp