Harden faction diplomacy authority

[kaiserproger]

Summary

• Require server-side faction leader authority for diplomacy status changes, treaties, team edits, and team removals.
• Validate messenger ownership/targets and treaty participants before starting or answering messenger flows.
• Recompute team edit costs server-side and bound faction settings such as recruit caps and colors.

Verification

• ./gradlew compileJava passed before commit.
• code-simplifier cleanup pass completed.
• code-reviewer final pass reported no findings.