fix: preserve owned lifetime of pool transactions

[torchello]

sqlx::Pool::begin() returns an owned Transaction<'static, DB>, but the wrapper shortens that lifetime to the borrow of &self. This prevents returning a transaction from a helper or storing it in an owned guard, even though its connection already belongs to the transaction.

Preserve SQLx's 'static return lifetime. The implementation and transaction behavior remain the same. A regression test requires the owned return type at compile time, drops the originating pool handle, then executes a query and rolls back successfully.

Validation (Rust 1.97.1, SQLx 0.9):

• cargo test --features "sqlite,runtime-tokio" --test sqlite — 117 passed.
• cargo clippy --all-targets --all-features -- -D warnings — passed.
• cargo fmt --all -- --check — passed.