ci: add GitHub Actions workflow and ESLint v9 config

Adds two-job CI (backend + frontend) running ruff, pytest (unit only -
skipping heavy ML deps), docker compose config validation for both dev
and prod overlays, plus npm ci -> eslint -> tsc -> vite build for the
frontend.

ESLint config uses the v9 flat-config format that the project was
already on (eslint v9 dropped .eslintrc support); replaces the broken
'eslint . --ext' invocation and adds @typescript-eslint, react-hooks,
and react-refresh plugins.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/eslint.config.js

@@ -0,0 +1,56 @@
+import js from "@eslint/js";
+import tsParser from "@typescript-eslint/parser";
+import tsPlugin from "@typescript-eslint/eslint-plugin";
+import reactHooks from "eslint-plugin-react-hooks";
+import reactRefresh from "eslint-plugin-react-refresh";
+
+export default [
+  { ignores: ["dist", "node_modules", "public", "*.config.js", "*.config.ts"] },
+  js.configs.recommended,
+  {
+    files: ["src/**/*.{ts,tsx}"],
+    languageOptions: {
+      parser: tsParser,
+      parserOptions: {
+        ecmaVersion: "latest",
+        sourceType: "module",
+        ecmaFeatures: { jsx: true },
+      },
+      globals: {
+        window: "readonly",
+        document: "readonly",
+        console: "readonly",
+        navigator: "readonly",
+        localStorage: "readonly",
+        setTimeout: "readonly",
+        clearTimeout: "readonly",
+        crypto: "readonly",
+        KeyboardEvent: "readonly",
+        Event: "readonly",
+        HTMLInputElement: "readonly",
+        HTMLButtonElement: "readonly",
+        HTMLDivElement: "readonly",
+        HTMLParagraphElement: "readonly",
+        HTMLSpanElement: "readonly",
+      },
+    },
+    plugins: {
+      "@typescript-eslint": tsPlugin,
+      "react-hooks": reactHooks,
+      "react-refresh": reactRefresh,
+    },
+    rules: {
+      ...tsPlugin.configs.recommended.rules,
+      ...reactHooks.configs.recommended.rules,
+      "react-refresh/only-export-components": ["warn", { allowConstantExport: true }],
+      "no-unused-vars": "off",
+      "@typescript-eslint/no-unused-vars": [
+        "warn",
+        { argsIgnorePattern: "^_", varsIgnorePattern: "^_" },
+      ],
+      "@typescript-eslint/no-explicit-any": "off",
+      "no-undef": "off",
+      "no-empty": ["error", { allowEmptyCatch: true }],
+    },
+  },
+];