Add release workflow, LICENSE, and version stamping

Tag-triggered workflow builds multi-platform binaries, pushes a multi-arch
Docker image to GHCR, bumps and releases the Helm chart, and creates a
GitHub release with all binary artifacts. Adds GPL-3.0 LICENSE and version
variable stamped at build time via ldflags.

.gitignore

@@ -0,0 +1,27 @@
+# build output
+/terdut
+/terdut-server
+
+# SQLite database files
+*.db
+*.db-shm
+*.db-wal
+
+# editor / OS
+.DS_Store
+*.swp
+*.swo
+*~
+
+# IDE
+.idea/
+.vscode/
+*.iml
+
+# local tooling
+.claude/
+.graymatter/
+
+# Helm dependency downloads
+charts/terdut-server/charts/
+charts/terdut-server/Chart.lock