test(signer): isolate integration tests with TEST_DATABASE_URL

[sergey3bv]

Summary

• Centralize signer DB setup in integration_test_db;
• Wire permission, client-pubkey and signer daemon integration tests through it;
• Updating CI and documenting new behaviour.

Avoid naming corporate partners, customers, or other sensitive external brands in this PR title or body. Use generic descriptors unless a maintainer explicitly approves the public reference.

Motivation

• Improving developer experience

Related Issue

• Closes feat: isolate signer integration tests from the development database #220

Testing

• cargo test --workspace --verbose
• cargo clippy --workspace --all-targets --all-features -- -D warnings -A deprecated
• cargo fmt --all -- --check
• Manual verification completed

Visuals

• UI/web change with screenshots/video attached
• No visual change
• Visuals and text avoid sensitive external brand or partner names unless explicitly approved