refactor: trait abstractions for testable DB layer

[bitalizer]

Summary

• Define Extractor and Inserter traits behind which DatabaseExtractor and DatabaseInserter implement their DB operations
• Make DatabaseMigrator, TableMigrator, and ConstraintsCreator generic over these traits, enabling mock-based unit testing of orchestration logic without real databases
• stream_rows returns BoxStream<'static> via async_stream::stream!, owning its pooled connection inside the stream
• Remove direct pool access (open_row_stream, pub pool) — all DB interaction goes through trait methods
• Bump version to 0.3.0, edition to 2024

What changed

File	Change
src/extract/traits.rs	New Extractor trait
src/insert/traits.rs	New Inserter trait
src/extract/extractor.rs	Implements Extractor, pool now private, stream_rows rewritten
src/insert/inserter.rs	Implements Inserter
src/migrate/migrator.rs	DatabaseMigrator<E, I> generic
src/migrate/table_migrator.rs	TableMigrator<E, I> generic, uses stream_rows
src/migrate/constraints_creator.rs	ConstraintsCreator<I> generic
src/main.rs	let mut migrator → let migrator
Cargo.toml	version 0.3.0, edition 2024, add async-stream
Various	cargo fmt import reordering (edition 2024)

What does NOT change

• Concurrency model (semaphore + spawned tasks + fail-fast via watch channel)
• Row batching logic (packet size limits, INSERT construction)
• Constraint creation (best-effort, errors logged)
• All existing 113 tests pass

Test plan

• cargo build — compiles cleanly
• cargo clippy -- -D warnings — no warnings
• cargo fmt --check — formatted
• cargo test — 113 tests pass
• Smoke test against real MSSQL → MySQL migration (recommended before merge)