fix: unbreak Backend CI (validation + archiving compile errors)

[abdulwaarith0]

Backend CI is red on main: cargo test --workspace doesn't compile, so no tests run and every open backend PR inherits the red. A few crates were merged non-compiling. This fixes two of them; the rest is tracked in #1180.

validation:

• The async redis calls (get/incr/expire/set_ex) need the AsyncCommands trait in scope. It wasn't imported.
• Arc is used by ValidationWebSocketHandler but std::sync::Arc wasn't imported.
• position_cache is mutated through &self (the validator is shared via Arc), so a plain HashMap field can't work. Wrapped it in a Mutex with short locked sections that don't span awaits, and fixed a borrow-of-moved in parse_uci_move (compute is_castling before moving from_square/to_square into the struct).

archiving:

• reqwest needs its multipart feature for the Arweave upload.
• cost_usd is Option<f64> but was assigned an unwrapped f64 in two branches.
• metadata.tags is a Vec, not an Option.
• form_data mixes literal and formatted keys, so it needs String keys.
• a format string had an unbalanced brace.

Verified locally: both crates compile and cargo test -p validation / -p chess pass. This doesn't fully green CI on its own (the metrics crate, see #1180, still needs fixing), but it clears two of the blockers.

[abdulwaarith0]

On the red CI here: this clears the validation and archiving compile errors, but main also has the metrics crate not compiling on its own (it calls .with_label_values(...) on plain Counter/Histogram). So the workspace still won't fully build until metrics is fixed too, and this PR's CI will stay red until then.

I left metrics out because the fix is a labeled-metrics design choice (CounterVec/HistogramVec + label names) that's really yours to make. The full picture and repro is in #1180.