feat: phase 3 of AI pipeline hardening — extraction quality

Replace stateless one-shot extraction with a stateful, paced, cancellable
pipeline. Six subtasks:

- 3.1 Sentence-aware chunking with 800-char overlap (was paragraph-only
  at 4000 chars). Hard-split fallback for runaway sentences.
- 3.2 Stateful extraction: chunks 2+ receive an "already-extracted topic
  IDs" hint capped at 200 IDs, so the model reuses IDs instead of
  inventing variants like software-developer vs software-engineer.
- 3.3 Token-bucket limiter in llmRetry.js (extractionLimiter, 5 req/min).
  callLLM awaits the limiter before fetch; 429+Retry-After calls
  pauseUntil. Replaces hard setTimeout(12000) and setTimeout(15000).
- 3.4 relevance_locked column on topics — admin edits to relevance are
  sticky across re-extraction. Migration + merge respects the flag +
  unlock checkbox in KnowledgeGraph edit form.
- 3.5 Unify relation vocabulary — handbook prompt no longer mentions
  legacy "executes"; one-shot migration rewrites existing executes rows
  to executed_by with source/target swapped.
- 3.6 Cancellation — Cancel button on UploadZone wired to an
  AbortController threaded into callLLM; aborted runs persist status =
  "cancelled" rather than "failed".

Tests: 16 new unit tests for chunkText, buildKnownIdsHint, and
createLimiter. All 61 tests pass, 0 lint errors, build clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pb_migrations/1780500000_updated_topics_relevance_locked.js

@@ -0,0 +1,23 @@
+/// <reference path="../pb_data/types.d.ts" />
+migrate((app) => {
+  const collection = app.findCollectionByNameOrId("pbc_2800040823")
+
+  // add field — relevance_locked is set to true whenever an admin edits
+  // learning_relevance via the UI; mergeKnowledgeGraph must never overwrite
+  // learning_relevance on a locked topic during re-extraction.
+  collection.fields.addAt(5, new Field({
+    "hidden": false,
+    "id": "bool_relevance_locked",
+    "name": "relevance_locked",
+    "presentable": false,
+    "required": false,
+    "system": false,
+    "type": "bool"
+  }))
+
+  return app.save(collection)
+}, (app) => {
+  const collection = app.findCollectionByNameOrId("pbc_2800040823")
+  collection.fields.removeById("bool_relevance_locked")
+  return app.save(collection)
+})

pb_migrations/1780500001_normalize_relation_types.js

@@ -0,0 +1,43 @@
+/// <reference path="../pb_data/types.d.ts" />
+// One-shot data migration: rewrite legacy "executes" relations to the
+// canonical "executed_by" vocabulary by swapping source and target.
+// Previously `role --executes--> process`; canonical is
+// `process --executed_by--> role`.
+migrate((app) => {
+  const records = app.findRecordsByFilter(
+    "pbc_1883724256",  // relations collection
+    'type = "executes"',
+    '',
+    0,
+    0,
+  )
+
+  for (const rec of records) {
+    const source = rec.get("source")
+    const target = rec.get("target")
+    rec.set("type", "executed_by")
+    rec.set("source", target)
+    rec.set("target", source)
+    app.save(rec)
+  }
+}, (app) => {
+  // Reverse: turn executed_by back into executes (best-effort — only those
+  // created before this migration would have been "executes"; rolling back
+  // will affect any newer executed_by rows too).
+  const records = app.findRecordsByFilter(
+    "pbc_1883724256",
+    'type = "executed_by"',
+    '',
+    0,
+    0,
+  )
+
+  for (const rec of records) {
+    const source = rec.get("source")
+    const target = rec.get("target")
+    rec.set("type", "executes")
+    rec.set("source", target)
+    rec.set("target", source)
+    app.save(rec)
+  }
+})