fix(BUY-63030): route MCP catalog reads through catalogDb; surface unavailable on zero-count category fallback

[BuyWhere]

Context

Parent: BUY-63027 (SEV-1 MCP fabricated cache regression). BUY-63030 is the deploy-and-verify subtask.

Live MCP probe at 2026-07-17T03:24Z showed:

• search_products(laptop, SG/US) returned rows but polluted cross-market results
• get_deals(SG/US) returned real rows
• list_categories(SG/US) still served cached static zero-count categories with unavailable:false

Root cause

MCP catalog reads were routing through the primary maglev DB (the catalog ingestion primary), which has been returning the fabricated/stale total rows seen in BUY-62788 / BUY-63027. The catalogDb pool already exists in mcp-railway/src/config.ts (BUY-53789, pointed at the canonical ~127M-row catalog replica), but src/routes/mcp.ts was never switched to use it for catalog reads.

Local patch

• Import catalogDb from ../config.
• Route MCP catalog reads through catalogDb for: search_products, get_product, compare_products, get_deals, list_categories, find_best_price, probeDiscountPctColumn, and the regional FTS fallback in get_deals.
• Keep ingestion/auth/health writes on primary db (write-path correctness, liveness checks).
• list_categories already recomputes unavailable from cached rows so pre-fix cache payloads now surface unavailable:true for zero-count static fallbacks.

Test plan

After merge + deploy, expect list_categories to return meta.unavailable:true until cache TTL clears AND mcp_category_summary_by_country is populated; then nonzero counts with unavailable:false.