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/schemas.py

@@ -288,12 +288,13 @@ class KbTreeFolder(BaseModel):
     resolves it with ``from __future__ import annotations``).
 
     ``summary_pending`` (phase 98, D3 — ONE concept): true iff this
-    folder's recursive count ≥ ``MIN_DOCS_PER_FOLDER`` (2) AND it has
+    folder's recursive count ≥ ``MIN_DOCS_PER_FOLDER`` (1) AND it has
     NO stored ``folder_summaries`` row (AI or manual — any row) —
     exactly the candidate the sync-time gap-fill regenerates (the
     :func:`app.rag.folder_summaries.missing_folder_summaries` set).
-    A < 2-document folder is never pending (it never gets a summary —
-    its one file line IS its description).
+    A 0-document folder cannot exist (a folder is a catalogue prefix
+    only), so every existing folder with no stored row is pending —
+    single-file folders included.
     """
 
     kind: Literal["folder"] = "folder"
@@ -325,12 +326,13 @@ class KbTreeSource(BaseModel):
 
     ``summary_pending`` (phase 98, D3 — ONE concept): true iff the
     source's recursive ``documents`` count ≥ ``MIN_DOCS_PER_FOLDER``
-    (2) AND no stored ``(source, "")`` row (AI or manual — any row) —
+    (1) AND no stored ``(source, "")`` row (AI or manual — any row) —
     exactly the source-root candidate the sync-time gap-fill
     regenerates (the
     :func:`app.rag.folder_summaries.missing_folder_summaries` set).
     A registered 0-document source is never pending (there is nothing
-    to summarize).
+    to summarize); a single-document source IS pending while its root
+    row is absent.
     """
 
     name: str
@@ -353,7 +355,7 @@ class KbTree(BaseModel):
 
     Every SOURCE and FOLDER node carries ``summary_pending`` (phase
     98, D3): true iff its recursive document count ≥
-    ``MIN_DOCS_PER_FOLDER`` (2) AND it has no stored ``folder_summaries``
+    ``MIN_DOCS_PER_FOLDER`` (1) AND it has no stored ``folder_summaries``
     row — exactly ``missing_folder_summaries``'s candidate set (the
     marker never drifts from the gap-fill); FILE nodes carry no flag
     (the file table has no description column).