feat(rag): summarize single-document folders (MIN_DOCS_PER_FOLDER 2 → 1)

Relax the phase-94 folder-summary scope rule from ≥ 2 documents to
≥ 1: a folder (or source root) is a candidate while ANY document
lives under it, so single-file folders and single-file source roots
get their own lite-written description. A row is now pruned only
when its folder loses its last document (vanishes from the
catalogue).

The constant is the single source of truth, so the flip propagates
to the generator's candidate set, the prune pass, the
missing_folder_summaries gap probe (the next sync self-heals the
new gaps), and the KB-tree summary_pending markers (1-doc folders /
sources now read "Summary pending" until their row lands).

Docstrings/comments across app/, scripts/import_docs.py, and the
E2E fixtures updated to the ≥ 1 wording. Unit + integration tests
updated to the new semantics (the pruned-below-minimum scenario is
now a folder losing its LAST doc; single-doc folders are pinned as
candidates/pending). Full suite: 2314 passed, app coverage 99%;
ruff + pyright clean; folder-summary E2E stories pass in isolation
(ls_tree_drilldown, sync_summary_visibility, kb_tree,
kb_tree_nav, document_dates, oneshot_llm_retry).

app/api/sync.py

@@ -353,7 +353,8 @@ async def _run_sync() -> None:
         # changed KB (added + updated > 0) is a full regeneration
         # (today's behavior, byte-identical); an unchanged re-sync
         # takes the GAP probe instead of the old table-empty one — a
-        # candidate folder (at least 2 docs) with no stored row: a gap
+        # candidate folder (at least 1 doc — every existing folder)
+        # with no stored row: a gap
         # fills ONLY the missing rows (only_missing=True — the
         # subsumed table-empty first-sync trigger included, where
         # every candidate is missing), a complete table burns zero