feat: private/custom git repos - GitHub, GitLab, Bitbucket, custom

- Add repository URL validation for GitHub, GitLab, Bitbucket, and custom hosts
- Add git provider layer (listDirectory, downloadRawFile) for all providers
- Wire githubJsonService and scriptDownloader to use provider; sync/download from any supported source
- Update GeneralSettingsModal placeholder and help text; .env.example and env schema for GITLAB_TOKEN, BITBUCKET_APP_PASSWORD

Closes #406

src/server/lib/gitProvider/github.ts

@@ -0,0 +1,60 @@
+import type { DirEntry, GitProvider } from './types';
+import { parseRepoUrl } from '../repositoryUrlValidation';
+
+export class GitHubProvider implements GitProvider {
+  async listDirectory(repoUrl: string, path: string, branch: string): Promise<DirEntry[]> {
+    const { owner, repo } = parseRepoUrl(repoUrl);
+    const apiUrl = `https://api.github.com/repos/${owner}/${repo}/contents/${path}?ref=${encodeURIComponent(branch)}`;
+    const headers: Record<string, string> = {
+      Accept: 'application/vnd.github.v3+json',
+      'User-Agent': 'PVEScripts-Local/1.0',
+    };
+    const token = process.env.GITHUB_TOKEN;
+    if (token) headers.Authorization = `token ${token}`;
+
+    const response = await fetch(apiUrl, { headers });
+    if (!response.ok) {
+      if (response.status === 403) {
+        const err = new Error(
+          `GitHub API rate limit exceeded. Consider setting GITHUB_TOKEN. Status: ${response.status} ${response.statusText}`
+        );
+        (err as Error & { name: string }).name = 'RateLimitError';
+        throw err;
+      }
+      throw new Error(`GitHub API error: ${response.status} ${response.statusText}`);
+    }
+
+    const data = (await response.json()) as { type: string; name: string; path: string }[];
+    if (!Array.isArray(data)) {
+      throw new Error('GitHub API returned unexpected response');
+    }
+    return data.map((item) => ({
+      name: item.name,
+      path: item.path,
+      type: item.type === 'dir' ? ('dir' as const) : ('file' as const),
+    }));
+  }
+
+  async downloadRawFile(repoUrl: string, filePath: string, branch: string): Promise<string> {
+    const { owner, repo } = parseRepoUrl(repoUrl);
+    const rawUrl = `https://raw.githubusercontent.com/${owner}/${repo}/${encodeURIComponent(branch)}/${filePath}`;
+    const headers: Record<string, string> = {
+      'User-Agent': 'PVEScripts-Local/1.0',
+    };
+    const token = process.env.GITHUB_TOKEN;
+    if (token) headers.Authorization = `token ${token}`;
+
+    const response = await fetch(rawUrl, { headers });
+    if (!response.ok) {
+      if (response.status === 403) {
+        const err = new Error(
+          `GitHub rate limit exceeded while downloading ${filePath}. Consider setting GITHUB_TOKEN.`
+        );
+        (err as Error & { name: string }).name = 'RateLimitError';
+        throw err;
+      }
+      throw new Error(`Failed to download ${filePath}: ${response.status} ${response.statusText}`);
+    }
+    return response.text();
+  }
+}