fix(hub): support GITHUB_TOKEN secret fallback for remote template imports

[chiefkarlin]

This PR resolves an authentication failure when importing remote templates from private GitHub repositories.

Changes

• Reads project-scoped GITHUB_TOKEN from the secret backend (or database fallback) in pkg/hub/template_bootstrap.go.
• Passes the retrieved token to FetchRemoteTemplate to authenticate download and git operations.
• Adds an offline unit test TestImportTemplatesFromRemote_WithProjectGithubToken to verify the secret resolution and fallback path using a mock HTTP client.

Closes #258

[gemini-code-assist]

Code Review

This pull request introduces a fallback mechanism to retrieve a project's GITHUB_TOKEN from either the secret backend or the database when importing templates from a remote source, and adds a corresponding unit test. The feedback recommends using the server's dedicated templateLog instead of the global slog for consistency, and addresses potential data races and test flakiness in the new unit test by protecting shared state with a mutex and restricting the mocked HTTP transport.