chore(license): migrate RouteForge metadata and docs to AGPL-3.0-or-later

[DeepZone]

Motivation

• Move project metadata and documentation from the permissive MIT stance to the GNU Affero General Public License v3.0 (AGPL-3.0-or-later) so network-deployed modifications must make source available to users.
• Add clear SPDX headers and contributor licensing guidance to reduce ambiguity about the project's license surface.
• Keep all functional, runtime and API behavior unchanged while ensuring build/test workflows remain green.

Description

• Set license = "AGPL-3.0-or-later" in backend/pyproject.toml.
• Added `

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Codex Task