Backend Phase A: foundation, migrations, Docker setup

- Go backend skeleton: config (caarlos0/env), slog JSON logging,
  pgxpool store, HTTP server with graceful shutdown.
- httpapi: render helpers, Problem errors, middleware chain
  (requestID / logging / recover / CORS), /healthz and /readyz.
- Migrations: full initial schema (users, sessions, lists,
  list_members, items, op_log SOURCE OF TRUTH, item_names) +
  golang-migrate runner binary using source/iofs (embedded).
- Docker: multi-stage Dockerfile (Go 1.26 -> distroless nonroot),
  builds both server and migrate binaries.
- deploy: docker-compose (caddy + backend + migrate + postgres:16),
  Caddyfile (auto-HTTPS), .env.example, pg extensions init script.
- AGENTS.md: project context + roadmap for AI agents.

Verified: image builds, both binaries run in container (smoke test).

.gitignore

@@ -0,0 +1,55 @@
+# === Secrets / local env ===
+.env
+.env.*
+!.env.example
+deploy/.env
+*.pem
+*.key
+
+# === Go ===
+backend/bin/
+/backend/server
+/backend/migrate
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+*.test
+*.out
+/backend/coverage/
+
+# === sqlc generated (keep if committed; uncomment to ignore) ===
+# backend/internal/store/sqlc/
+
+# === IDE / OS ===
+.idea/
+.vscode/
+*.iml
+.DS_Store
+Thumbs.db
+
+# === Agent / tooling local data ===
+.zcode/
+
+# === Docker ===
+deploy/data/
+
+# === Android / Gradle ===
+android/.gradle/
+android/build/
+android/app/build/
+android/local.properties
+android/captures/
+android/.cxx/
+*.apk
+*.aab
+*.ap_
+*.dex
+*.keystore
+*.jks
+keystore.properties
+
+# === Gradle wrapper (KEEP gradlew + wrapper jar; ignore caches) ===
+android/**/build/
+!android/gradle/wrapper/gradle-wrapper.jar