Pin chrono <0.4.40 to fix arrow-arith build conflict

[geoffjay]

Summary

• Pin chrono to >=0.4.38, <0.4.40 in workspace Cargo.toml to avoid error[E0034]: multiple applicable items in scope caused by chrono 0.4.40+ adding Datelike::quarter() which conflicts with arrow-arith 53.4.0's ChronoDateExt::quarter()
• Centralize chrono as a workspace dependency (with serde feature) so all crates use a consistent, pinned version
• All 9 crate-level Cargo.toml files updated to reference chrono = { workspace = true }

Context

arrow-arith 53.4.0 (pulled in by lancedb 0.16 → datafusion → arrow) defines a ChronoDateExt trait with a quarter() method. Starting in chrono 0.4.40, the Datelike trait also gained a quarter() method, causing ambiguity. The latest arrow-arith (58.x) fixes this but we're pinned to 53.x by lancedb 0.16.

Test plan

• cargo clippy --workspace passes without errors
• cargo test -p orchestrator passes
• All workspace tests pass (one pre-existing doctest failure in hook/config.rs unrelated to this change)

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