feat: add daily activities counter

[ulemons]

PR: Add activities_daily_counts.pipe and optimize with PREWHERE

What

• Add a Tinybird pipe to compute daily per-segment unique activity counts required by the cache-worker as part of deprecating QuestDB.
• Optimize the query by using PREWHERE on segmentId, timestamp (and platform when present) so the engine reads only filter columns first, discards non-matching rows early, and then loads activityId only for survivors.

Why

• The cache-worker needs a canonical time series of daily counts per segment to replace the equivalent queries previously served by QuestDB.
• Using PREWHERE reduces scanned bytes, decompression, and memory pressure during uniq* aggregation—especially when filters are selective (not “high cardinality”, but high selectivity).

Checklist ✅

• Label appropriately with Feature, Improvement, or Bug.

[borfast]

TIL about PREWHERE 😁
👍