SPDX headers missing trailing period in all files

[VasilevNStas]

Problem

Every SPDX header in the repository is missing the trailing period after APN and MIT:

// SPDX-FileCopyrightText: Copyright (c) 2026 APN
// SPDX-License-Identifier: MIT

According to the REUSE specification, SPDX headers should include a trailing period.

Fix

Add a period to the end of every SPDX header line across:

• .gitignore
• REUSE.toml
• auth.proto
• ping.proto
• pong.proto
• welcome.proto

No wire-format change — comments only.

[VasilevNStas]

@davvd plz review