planner/optimizer: consider switching time units from seconds to milliseconds #416

planner/optimizer — `solution.rs` and related files (`aqe_extractor.rs`, `candidate_gen.rs`, etc.) use `u64` seconds throughout for repeat intervals and window sizes (`t_repeat_secs`, `min_t_repeat_secs`, `t_repeat_gcd_secs`, window/slide sizes).

Question raised in review: should these switch to milliseconds for finer granularity (e.g. sub-second scrape intervals or window sizes)? This is a cross-file unit change with real diff risk, so it needs its own scoped pass rather than a quick fix — and a decision on whether second-granularity is actually a limitation in practice before doing the work.

Found during code review of PR #407.