[Enhancement] tblActivityLog: add (shortCode, createdAt) composite index; decide partitioning

[Salem874]

Problem

The dominant analytics query is per-short-code over a date range, but tblActivityLog only has single-column indexes; monthly partitioning is a commented suggestion, not enabled (confirms earlier audit #12).

Fix

Add IDX_log_shortcode_created (shortCode, createdAt); review whether standalone IDX_log_shortcode/IDX_log_org remain justified given write cost. Decide on monthly partitioning before analytics ships.

Relates to analytics issues #41/#42.

Acceptance criteria

• Composite index added; per-code date-range query uses it.
• Partitioning decision documented.

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From the 2026-06-04 schema review — see docs/SCHEMA_REVIEW_2026-06-04.md.