bug(fts): D1 FTS5 upsert failures observed for diff rows across all repos

[liplus-lin-lay]

observation

Cloudflare Workers logs (2026-04-26 cron run) show repeated `Failed to upsert FTS5 row for diff ...` errors across all 5 polled repos:

```
Failed to upsert FTS5 row for diff Liplus-Project/dipper_ai@07391bc
Failed to upsert FTS5 row for diff Liplus-Project/liplus-language@ce3b9d5 (multiple)
Failed to upsert FTS5 row for diff Liplus-Project/liplus-desktop@add899f
Failed to upsert FTS5 row for diff 770d7a9
Failed to upsert FTS5 row for diff Liplus-Project/github-webhook-mcp@1d237f4
```

Pattern: cross-repo (not specific to one repo), specifically on diff type rows. Long-standing per Master's framing.

scope

• Affects `pollDiffs` → `processAndUpsertCommitDiff` path
• D1 FTS5 mirror upsert fails; primary Vectorize upsert may still succeed (dual-store design)
• Search recall on diff type queries may degrade (sparse side missing rows)

fix direction (memo)

Investigation needed to determine:

• Specific D1 error (constraint violation? schema mismatch? rate limit?)
• Whether the failure is per-call or per-row
• Whether retry / fallback policy in `pipeline.ts` masks the failure level

The error message catches all upsert exceptions in `processAndUpsertCommitDiff` so root cause requires log-level enrichment or local repro.

related

• 既存 `processAndUpsertCommitDiff` 内 try/catch 包囲 (`src/pipeline.ts`)
• `src/fts.ts` upsertFtsRow / D1 FTS5 schema (`migrations/0001_fts5_init.sql`, `0002_fts5_rebuild.sql`)

[liplus-lin-lay]

PR #137 (chore(fts): enrich D1 upsert failure logs to surface root cause) は本 issue の root-cause fix ではなく Option B (log enrichment) のみのため、issue は open のまま継続。

PR body も Refs #135 で Closes 指定なしで作成したが、gh issue develop 由来の Development panel branch link が PR merge 時に自動 close を引いたため reopen。

次の :30 UTC cron iteration（pollDiffs）以降のログで FTS5 diff upsert detail (#135): 行に出る errorName / tokenizerKind / contentChars / filePathChars を観測してから、root-cause fix を別 PR で行う。