fix(security): load translator key from environment (#601)

[gHashTag]

The Coptic translator contained a hardcoded DeepSeek credential. It now reads DEEPSEEK_API_KEY from the environment and fails with a clear configuration error when the value is missing, empty, or whitespace, before creating output or making network requests.

References #601. The security incident remains open: removing a credential from the current source does not establish that it was revoked or remove it from Git history. This PR does not test the exposed credential or change account credentials.

Validation:

• Python syntax compilation passed.
• Offline runtime checks passed for missing, empty, and whitespace environment values; each fails before any output or network request.
• With a dummy environment value, the request reaches a mocked urlopen with the expected Authorization header and response handling. No actual API call was made.
• Targeted review confirmed only the credential configuration changed; the exposed credential pattern is absent from the updated file. git diff --check passed.
• zig build and zig build test were attempted but cannot pass with the locally installed Zig 0.16.0 (CI pins 0.15.2): build-script/dependency API errors include linkLibC, trimLeft, and getEnvVarOwned. No Zig source changed.