feat(snippets): reverse lookup — find snippets by repo/path/symbol

"What canonical helpers already live in this file?" was unanswerable:
location lived only in the body markdown. It is now a jsonpath containment
query over the `notes.data` mirror added by migration 0070.

- One predicate in two dialects in services/knowledge.py: SQL (`data @?`,
  applied in the browse arm and the keyword arm before count/pagination, so
  totals stay honest) and Python (`location_matches`, for the semantic arm
  which post-filters candidates it already holds). Both must change together.
- Parts are ANDed within a SINGLE locations entry — repo A in one entry and
  path B in another is not "recorded at A/B". `path` also matches as a
  directory prefix, via jsonpath `starts with` rather than `@>`, which the
  same GIN index serves.
- `repo`/`path`/`symbol` reach the service, the REST list and the MCP tool
  under one name with one default (rule #33); the MCP docstring teaches the
  place form, and so does the reusing-code skill (plugin.json bumped).
- UI: a Location disclosure beside the snippet search, with its own empty
  state — "nothing kept there, so what you're about to write is new."

Settles #2083's open question (pre-0070 NULL `data`) by backfilling after
all: `backfill_snippet_data` runs at startup, deriving the mirror from the
body with the same parser the read path trusts. 0070's caution was about
mangling a hand-edited body; this never touches the body. The alternative
was a permanent second body-regex arm, or a query that silently answers
"nothing here" for an old snippet and gets the helper written twice.

Refs #2083, milestone #232.

src/scribe/app.py

@@ -170,6 +170,15 @@ def create_app() -> Quart:
                 await backfill_note_embeddings()
             except Exception:
                 logger.warning("Embedding backfill failed", exc_info=True)
+            # Snippets written before migration 0070 have no `notes.data` mirror,
+            # and the location reverse lookup queries that column — an unfilled
+            # row would read as "no snippet here" rather than as a gap. Separate
+            # try block so neither backfill can skip the other.
+            try:
+                from scribe.services.snippets import backfill_snippet_data
+                await backfill_snippet_data()
+            except Exception:
+                logger.warning("Snippet data backfill failed", exc_info=True)
 
         asyncio.create_task(_delayed_backfill())