fix(routing): total_pages=0 document passes routing validation and corrupts token-reduction estimate #279

Summary

route_query() checks fallback_only on each document but does not check total_pages. A document with total_pages=0 (race condition between ingest completion and route call, or a data integrity gap) passes all guards, enters routing, and returns estimated_token_reduction='0%' — which is misleading. Worse, the router module may perform integer arithmetic with total_pages=0 causing division-by-zero or negative page-range comparisons.

Root Cause

backend/app/api/routing.py around line 99:

total_pages = sum(d.total_pages for d in docs)   # can be 0
# ...
if any(d.fallback_only for d in docs):
    raise ...   # only fallback_only is checked, not total_pages=0

estimate_token_reduction(total_pages=0, ...) returns '0%'. The router module receives total_pages=0 and may compute section_coverage = selected_pages / total_pages → ZeroDivisionError.

Failure Scenario

1. Tenant uploads a PDF. Ingest starts but has not yet written total_pages to Postgres.
2. Tenant immediately routes a query against the document (race condition).
3. store.get_document(doc_id) returns DocumentRecord(total_pages=0, fallback_only=False).
4. fallback_only=False passes the guard.
5. total_pages=0 is passed to the router module.
6. Router computes coverage_ratio = len(selected_sections) / 0 → ZeroDivisionError (unhandled 500).
7. Or: returns estimated_token_reduction='0%' — caller believes all tokens are used, no reduction.

Proposed Fix

Add an explicit guard for total_pages=0:

for doc in docs:
    if doc.total_pages == 0:
        raise HTTPException(
            status_code=409,
            detail={
                "code": "DOCUMENT_NOT_READY",
                "document_id": doc.doc_id,
                "message": "Document is still being indexed. Retry in a few seconds.",
            }
        )
    if doc.fallback_only:
        raise ...

This gives callers a clear, retryable signal instead of a cryptic 500.