docs: add PLANNING.md v0.3 blueprint and .gitignore

Blueprint for a self-hosted LLM API relay gateway (M0-M4: skeleton,
users+keys, proxy, billing, conversion, channels). .gitignore excludes
DB files, node_modules, and build output.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,32 @@
+# Build artifacts
+dist/
+/target/
+*.exe
+*.test
+*.out
+
+# Dependencies
+node_modules/
+web/node_modules/
+
+# SQLite databases (contain API key ciphertext, password hashes)
+*.db
+*.db-wal
+*.db-shm
+data/
+server/data/
+
+# Local environment & secrets
+.env
+.env.*
+!.env.example
+
+# Logs
+*.log
+logs/
+
+# OS / editor
+.DS_Store
+Thumbs.db
+.idea/
+.vscode/