[Release] Configure release Firebase/Google services file

[jjoonleo]

Backlog ID: ISSUE-019
Source: plans/release_app_todos.md

Goal

Ensure release builds use the correct Firebase config.

Parallelization

Status: codex-ready

Prerequisites

• None

Acceptance criteria

• Release google-services.json source is defined.
• android/app/src/release/google-services.json or CI secret decode process is documented.
• Release config matches package club.devkor.ontime.
• Auth-related SHA-1/SHA-256 fingerprints are added after signing decisions are finalized.

Codex thread guidance

• Work only on this issue's scope.
• Do not modify unrelated release issues.
• If this issue is blocked by external access or human approval, prepare the needed checklist/text and leave the issue open.
• This issue is intended to be safe for an independent Codex thread now.