Migrate marketplace & signal stores to SQLite (issue #36)

[codewithzubair07]

Closes #36

Replaces full-file JSON reads for listings/offers/signals/replies with
an indexed SQLite schema (foreign keys: offers→listings, replies→signals).

• lib/sqlite-db.ts — schema + connection (node:sqlite, no native deps)
• lib/market-store.ts, lib/signal-store.ts — same exported function
  signatures, SQL-backed internals (no API route changes needed)
• scripts/migrate-json-to-sqlite.ts — idempotent migration from
  .data/*.json, verifies record-count parity, run via npm run migrate:sqlite

Out of scope (left on JSON): marketProfileViews, blockList — not
mentioned in the issue's acceptance criteria.

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