refactor: remove lite variant, fix release workflow

- Remove all lite/full variant logic from frontend, API, shared constants,
  docs, and tests (single unified Docker image only)
- Replace single QEMU multi-arch Docker build with per-architecture native
  builds (amd64 + arm64) and manifest merge to fix disk space exhaustion
- Add disk cleanup step and per-platform build cache scopes
- Switch release trigger from push to workflow_dispatch
- Add GitHub issue templates and PR template

apps/api/src/routes/settings.ts

@@ -6,7 +6,6 @@
  * GET  /api/v1/settings/:key — Get a specific setting
  */
 
-import { PYTHON_SIDECAR_TOOLS } from "@stirling-image/shared";
 import { eq } from "drizzle-orm";
 import type { FastifyInstance, FastifyReply, FastifyRequest } from "fastify";
 import { db, schema } from "../db/index.js";
@@ -27,10 +26,7 @@ export async function settingsRoutes(app: FastifyInstance): Promise<void> {
       settings[row.key] = row.value;
     }
 
-    const variant = process.env.STIRLING_VARIANT === "lite" ? "lite" : "full";
-    const variantUnavailableTools = variant === "lite" ? [...PYTHON_SIDECAR_TOOLS] : [];
-
-    return reply.send({ settings, variant, variantUnavailableTools });
+    return reply.send({ settings });
   });
 
   // PUT /api/v1/settings — Save settings (admin only)