docs: Replace GPL 2+ long text with SPDX License ID across core codebase

[ArthPatel1502]

Closes #4190. Implements an automated search-and-replace migration pattern across the codebase to cleanly substitute legacy multi-line GPL-2.0-or-later statements with inline 'SPDX-License-Identifier: GPL-2.0-or-later' declarations in-place.

Following the advice from @nilason and @echoix:

• Changes are executed in-place within the original license commentary positioning blocks.
• Complex multi-licensed or non-standard project files have been systematically skipped for manual isolation during phase two.
• Commits are structurally segmented by root architectural components (/lib, /db, /imagery) to maximize review efficiency.