Truncate stale logging entries (older than 2025-11-06) (#76)

Migration 003 cleared `logging` for entries older than 2024-01-01. The
DB has since grown to 17GB on the production instance, dominated by
build-log text. Anything older than ~6 months is unlikely to be
inspected and the retained log costs disk space + read cost on any
query that reads full row payloads (the latter mostly fixed by 005's
covering index, but disk space and table scans elsewhere still
benefit).

The cutoff (20251106000000) is fixed and approximately 6 months before
the current development date.

This UPDATE doesn't shrink the DB file. Freed page space is reused by
future inserts; to actually reclaim disk space, run VACUUM in a
maintenance window after this migration is deployed (it cannot run
inside the migration's transaction).

Verified locally: rows with build_dt before the cutoff have their
logging text replaced; newer rows untouched.

Author: mattgodbolt

migrations/006-truncate-stale-logs.sql

@@ -0,0 +1,27 @@
+--------------------------------------------------------------------------------
+-- Up
+--------------------------------------------------------------------------------
+
+-- Migration 003 cleared `logging` for entries older than 2024-01-01. Since
+-- then the DB has grown to ~17GB, dominated by the `logging` text column on
+-- `latest` rows. Anything older than ~6 months is unlikely to be inspected
+-- and the build log retained for it costs disk and per-row read cost
+-- (the latter mostly fixed by the covering index in 005, but the disk space
+-- and table scans elsewhere still benefit).
+--
+-- The cutoff is fixed; build_dt format is YYYYMMDDHHMMSS local time.
+--
+-- Note: this UPDATE doesn't shrink the DB file. Freed page bytes are reused
+-- by future inserts; to reclaim disk space, run VACUUM in a maintenance
+-- window after this migration is deployed (it's not part of the migration
+-- because VACUUM cannot run inside the migration's transaction).
+
+UPDATE latest
+   SET logging = 'Log eliminated due to age'
+ WHERE build_dt < 20251106000000
+   AND logging IS NOT NULL
+   AND logging != 'Log eliminated due to age';
+
+--------------------------------------------------------------------------------
+-- Down
+--------------------------------------------------------------------------------