fix: #36 add trailing period to all SPDX headers

[VasilevNStas]

Description

Add a trailing period to every SPDX header comment across all 6 files in the repo, per REUSE convention.

Related Issue

Fixes #36

Changes

File	Change
.gitignore	Period added to 2 SPDX lines
REUSE.toml	Period added to 2 SPDX lines
auth.proto	Period added to 2 SPDX lines
ping.proto	Period added to 2 SPDX lines
pong.proto	Period added to 2 SPDX lines
welcome.proto	Period added to 2 SPDX lines

Verification

• No wire-format changes — comments only
• All 12 SPDX lines updated uniformly

[VasilevNStas]

@yegor256 plz review