chore(deps): bump pq-sys and cron to latest

[pratyush618]

Summary

Two 0.x dependency bumps where the lockfile already maxed out within the existing range. Each lands as its own commit so either can be reverted independently.

Crate	Before	After	Notes
pq-sys	0.6	0.7	libpq bindings; postgres feature only
cron	0.13	0.16	Schedule::from_str + .after().next() API surface unchanged across 0.13→0.16

All other workspace deps (tokio, serde, diesel 2.x, redis 1.2, chrono, chrono-tz, libsqlite3-sys, uuid, crossbeam-channel, thiserror, log, openssl-sys, async-trait, base64, gethostname, tempfile) are already at their latest within the current ^ range; cargo update reports 0 packages to refresh. PyO3, rand, and thiserror major bumps are deferred to their own PRs (2b/2c/2d).

Verification (local)

• cargo check --workspace --features postgres — pq-sys 0.7.5 builds clean
• cargo test --workspace — 78 tests pass (43 unit + 1 storage + 5 + 29 across crates)
• cargo clippy --workspace --all-features -- -D warnings — clean
• cargo clippy --workspace --features postgres — clean
• cargo clippy --workspace --features redis — clean
• cargo fmt --all --check — clean

Test plan

• CI: SQLite, Postgres, Redis backend Rust suites green
• CI: lint job (clippy + fmt) green