[Bug] file_count is ~3% high due to external: import nodes being typed as 'file'

Follow-up to #945 / #952.

This is a follow-up to #945 (the original bug about wildly inflated file counts) and #952 (the fix for that bug). While reviewing #952, a smaller, separate issue was found — the file count is still slightly too high, just for a different reason.

 `node_type` defaults to `'file'` in the writer (`packages/core/src/repowise/core/pipeline/persist.py`) whenever no type is recorded, and rows representing third-party imports (id prefix `external:`, e.g. `external:node:path`) never have a type recorded — so they get counted as files.

 On a real index this was 79 rows out of 3,010 file-typed rows (~2.6%), confirmed by @swati510 in review on #952.

 Affects both:
 - `packages/server/src/repowise/server/routers/workspace.py` (workspace mode)
 - `packages/server/src/repowise/server/routers/stats.py` (single-repo mode, same `node_type == "file"` filter)

 Fix should either exclude `id LIKE 'external:%'` in both places, or (better) have the writer in `persist.py` record an explicit `node_type` for external-import nodes instead of leaving it to default. Should be fixed in both routers together to keep the two pages consistent. make it a .md file