Grouping the whole activities table by thread every run is unusable on
a large instance: on a 312 GB database it plans as a seq scan plus a
50M-row on-disk sort. Instead, walk activities in id order (flake ids
are time-ordered) from a persisted cursor up to the id that corresponds
to the deadline, and verify only the threads those activities belong to
through the (type, context) index. Measured on that instance: 147 ms
for a 50k-activity range scan, 349 ms warm / 14 s cold to verify ~700
threads.
The cursor is persisted per chunk so a killed run resumes. The
watermark walk keeps a cursor of its own so it cannot leave the
age-based walk stranded behind the deadline. Adds the retention_cursors
table.
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