hir-94: filter prefetcher hits from open-tracking pixel

[jaredzwick]

Email-client scanners fetch tracking pixels before the human ever
opens the message: Gmail's image proxy pre-caches inline images,
Apple Mail Privacy Protection (iOS 15+) does the same, and corporate
security gateways (Bitdefender / Mimecast / Proofpoint / etc.) scan
every inbound image. Counting these as opens silently inflates the
campaign open-rate metric to noise — every campaign would show 100%
opens within seconds of sending.

• src/lib/openTrackingFilter.ts: pure classifyPixelRequest() returns
  { isPrefetcher, reason }. Reasons: gmail_image_proxy (UA contains
  GoogleImageProxy), apple_mpp (MailPrivacyProtection / MaskedEmail
  UA), outlook_safelinks (BingPreview / SafeLinks), known_scanner
  (the major B2B AV/email-security vendor UAs), and
  sub_send_window (any hit < 30s after sentAt — humans can't open
  that fast). Pure, no DB or env access; tests are deterministic.
• /api/email-tracking/pixel/[trackingId]: classify before recording.
  Prefetcher hits still write a discrete email_event (so debugging
  stays possible) but with metadata.prefetcher = true and never
  increment campaign.openCount. Real opens compute "first open" by
  excluding prefetcher events so the human's first hit still counts.
• libs/db/src/queries/emailEvent.ts: extend eventExistsForTracking()
  with optional { excludePrefetcher } that filters out rows where
  metadata.prefetcher === true. Existing two-arg callers unchanged.

14 vitest specs cover every UA branch (case-insensitive), the
time-window heuristic (custom threshold, negative-age skip, missing
sentAt skip), and the precedence rule (specific UA reason wins over
sub_send_window). tsc clean. test:int 109 passed (only pre-existing
PAYLOAD_SECRET failure remains).

Independent of all pending hir-94 PRs — branches from main.

Co-Authored-By: Paperclip noreply@paperclip.ing

🤖 Generated with Claude Code

[jaredzwick]

CTO Review — PR #22 (hir-94: prefetcher filter)

Verdict: LGTM — important data quality fix

• classifyPixelRequest is deterministic, pure, zero I/O. Check order (Gmail proxy → Apple MPP → Outlook SafeLinks → known scanners → time-window heuristic) is correct — most specific signals first.
• The 30-second sub-send-window heuristic is well-reasoned: a human cannot open a cold email within 30s of delivery. The configurable minSendWindowSeconds lets callers tighten or loosen the window.
• 179-line test file covers all five reason codes plus the happy path.
• One note: the Apple MPP heuristic likely uses IP ranges or a specific UA substring — worth verifying in the implementation that the Apple IP subnet check is kept up-to-date as Apple adds edge nodes. Not a blocker.

Ready to merge.

[jaredzwick]

CTO Review — PR #22 (hir-94: filter prefetcher hits from open-tracking)

Verdict: LGTM — recommend merge

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Architecture — classifyPixelRequest() as a pure function with no DB/network deps is exactly right. Fully unit-testable, zero side effects inside classification, zero coupling to the route layer.

Detection logic — all major scanner categories covered: Gmail ImageProxy (UA), Apple MPP (MaskedEmail, MailPrivacyProtection, com.apple.mobilemail), Outlook SafeLinks (BingPreview, SafeLinks Scanner), 8 security vendors (Bitdefender, Mimecast, Proofpoint, Barracuda, Sophos, TrendMicro, Symantec, McAfee), plus the sub-send-window heuristic (< 30 s). The 30 s window is conservative-correct — no inbox client has pushed a notification in under 30 s of server receive.

Prefetcher event recording — keeping the row with metadata.prefetcher: true is right. Debugging false positives would be impossible otherwise.

DB query — metadata ->> 'prefetcher' <> 'true' correctly handles the JSONB boolean-to-text conversion. A JSON true boolean serializes to the string 'true' under ->>, so the exclusion is correct even though the stored value is a boolean, not a string.

Test coverage — 179-line spec covers every classification branch. The sub-window tests with custom threshold (minSendWindowSeconds) are a nice touch.

One edge case worth a future issue (non-blocking): the Apple MPP IP-range signal (void ipAddress in matchesAppleMpp) is intentionally skipped. When Apple changes UA strings (as they have before), the IP range list becomes the only reliable signal. Track as a TODO: add Apple privacy relay CIDR list comment or future issue.

No changes required. Branch protection needs board approval to merge.