Support loading authority secret key from environment variable

[average-gary]

Problem

The authority secret key is currently stored as plaintext in the TOML config file (e.g., pool-config.toml). This is a security risk because:

• The private key sits on disk during operation
• Anyone with file access can read the key
• Key may persist in backups, log files, or crash dumps

Related

• Related to Key-utils leak secret keys stratum#1958 (key-utils leak secrets)

Affected Components

• Pool (pool-apps/pool)
• Job Declaration Server (pool-apps/jd-server)
• Job Declaration Client (miner-apps/jd-client)

Note: Translator only requires the authority public key (no secret key needed).

Proposal

Add support for loading the authority secret key from an environment variable instead of the TOML config file:

• POOL_AUTHORITY_SECRET_KEY
• JDS_AUTHORITY_SECRET_KEY
• JDC_AUTHORITY_SECRET_KEY

Implementation approach:

1. Make authority_secret_key optional in the TOML deserialization struct
2. At config loading time, check for the corresponding environment variable
3. If env var is set, use it; otherwise fall back to TOML value (for backward compatibility)
4. Validate that either env var or TOML has the key (fail startup if neither)

This follows the existing pattern where RUST_LOG is already loaded from environment.

Backward Compatibility

TOML config files should continue to work. The env var takes precedence if both are set.

[plebhash]

I'd say this also falls under the scope of #280?