fix(server): close worker on shutdown and parse in_memory

[gnanirahulnutakki]

Summary

• close the server worker queue during graceful shutdown and join the worker thread
• stop channel forwarders from holding permanent worker sender clones
• accept explicit ARDUR_MEMORY=in_memory and add a unit test for the literal

Issues

• ARD-447
• ARD-449

Verification

• cargo check -p ardur-server — passed
• cargo test -p ardur-server — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test -p ardur-e2e-tests && cargo build --workspace --bins — passed

Signed-off-by: Ardur team@ardur.ai

[linear-code]

ARD-447

[gnanirahulnutakki]

Local verification and required GitHub checks are green. Merge is currently blocked by base branch policy; auto-merge is unavailable for this repo, and I am not bypassing branch protection with admin privileges from this session.

Verification:

• cargo check -p ardur-server — passed
• cargo test -p ardur-server — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo test -p ardur-e2e-tests && cargo build --workspace --bins — passed
• GitHub checks: DCO, macos-latest / stable, ubuntu-latest / stable — passed