fix(retention): dedupe contributor-decision-pack to latest-only, not a preserved series

[JSONbored]

The actual dominant cause of the recurring D1 crisis

While investigating #9432/#9435, I exported the live hosted D1 (8.96 GB / 10 GB cap) and measured per-table volume directly rather than trusting row counts alone. signal_snapshots came back at 6.35 GB — and within it, one signal type, contributor-decision-pack, accounts for 6.3 GB across 18,549 rows (~350 KB/row). That's 71% of the entire database's file size, and it long predates and dwarfs everything #9415 (merged) fixed — that PR bounded five other tables totaling well under 1 GB combined.

Root cause: a wrong doc comment

LATEST_ONLY_SIGNAL_SNAPSHOT_TYPES (src/db/retention.ts) explicitly excluded contributor-decision-pack, on the stated theory that it's "a bounded trend/change series by design" with volume "a fraction of" the three contributor-* types beside it that already dedupe (contributor-evidence-graph, contributor-outcome-history, contributor-strategy).

Both claims are false, verified directly:

• Not a series: src/services/decision-pack.ts's only reader calls listSignalSnapshots(env, CONTRIBUTOR_DECISION_PACK_SIGNAL, login))[0] — index [0], the exact same latest-only contract as its three neighbors. Nothing anywhere reads it as history.
• Not a fraction: measured live, it's ~750x the combined size of the three types it was assumed to dwarf.

Because signal_snapshots has a 90-day age-based retention rule (RETENTION_POLICY) but the hosted database itself is only ~65 days old, age-based pruning has not removed a single one of these rows yet — every one of the 18,549 rows is still live. The type has been accumulating ~700–1,300 rows/day since 2026-07-06. Left alone, it refills the 10 GB cap from empty in roughly 3–4 weeks on its own, regardless of #9415's fixes.

queue-health is the one signal type that genuinely is read as a series (maintainer-slop-duplicate-trend.ts shapes several weeks of snapshots into a trend card) — it correctly stays excluded, and this PR leaves it untouched.

Change

Move contributor-decision-pack into LATEST_ONLY_SIGNAL_SNAPSHOT_TYPES, correct the now-stale doc comments explaining the (wrong) exclusion, and update the tests that pinned the old (wrong) behavior:

• test/unit/retention.test.ts: the "preserves bounded history" test now covers only queue-health; a new dedicated test covers contributor-decision-pack deduping to latest-per-contributor; the contributor-intelligence-types test's decision-pack assertions flip from "series preserved" to "deduped."

No schema change, no new migration. retention.ts is fully covered by the updated tests (statements + branches).

What this doesn't fix

The already-accumulated 6.3 GB stays on disk until either (a) the next dedupeSignalSnapshots run prunes it (this fix makes that happen going forward), or (b) the file itself is reclaimed — D1 never runs VACUUM (cloudflare/workerd#1618), so freed pages are reused, not returned to the OS; the file size will plateau rather than shrink. A follow-up (tracked separately) covers reclaiming the already-inflated file via a fresh database + data migration.

Refs #9435

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[loopover-orb]

Important

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed · 🟨 Waiting}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.65%. Comparing base (7ed3531) to head (ae17e14).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9459      +/-   ##
==========================================
- Coverage   89.55%   88.65%   -0.91%     
==========================================
  Files         843      843              
  Lines      110073   110073              
  Branches    26194    26194              
==========================================
- Hits        98573    97582     -991     
- Misses      10238    11520    +1282     
+ Partials     1262      971     -291     

Flag	Coverage Δ
backend	93.62% <ø> (-1.65%)	⬇️

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Files with missing lines	Coverage Δ
src/db/retention.ts	100.00% <ø> (ø)

... and 3 files with indirect coverage changes