feat: add SQL-based cold storage backend

[prestwich]

Summary

• Adds signet-cold-sql crate with SQL-based ColdStorage implementations
• Unified SqlColdBackend using sqlx::Any supports both PostgreSQL and SQLite from a single implementation
• Database type auto-detected from connection URL (sqlite: or postgres:)
• All data stored in fully decomposed SQL columns for rich queryability
• Docker-based PostgreSQL test infrastructure with docker-compose.yml and scripts/test-postgres.sh
• Conformance tests pass against both SQLite (in-memory) and PostgreSQL (via Docker)

API

// SQLite
let backend = SqlColdBackend::connect("sqlite::memory:").await?;

// PostgreSQL
let backend = SqlColdBackend::connect("postgres://localhost/signet").await?;

Type aliases SqliteColdBackend and PostgresColdBackend are provided for backward compatibility.

Test plan

• cargo t -p signet-cold-sql --features test-utils — SQLite conformance passes
• ./scripts/test-postgres.sh — PostgreSQL conformance passes via Docker
• cargo clippy --all-features and --no-default-features — clean
• cargo +nightly fmt — clean

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