docs: add production MVP blueprint for real estate valuation app

[Moshbbab]

Motivation

• Provide a practical, production-oriented MVP blueprint to accelerate implementation of a real-estate valuation and buyer-matching platform.
• Capture architecture, data schema, API surface, valuation and matching logic, UX wireframes, deployment checklist, monetization strategy, and security guidance in a single reference.

Description

• Add a comprehensive design document at docs/real-estate-value-calculator-mvp.md covering system architecture, runtime components, and rationale.
• Include a simplified PostgreSQL schema for users, properties, market comps, valuations, buyers, matches, reports, subscriptions, and engagement events.
• Define v1 API routes (/api/v1) for auth, properties, valuations, comps, buyer matching, reports, billing, analytics, and admin; and outline frontend page structure and UI wireframes.
• Describe an explainable valuation engine (sales, income, cost approaches), rule-based buyer matching, deployment checklist, 14-day launch plan, monetization roadmap, security checklist, and scaling path, with placeholders for external providers and a required disclaimer.

Testing

• Ran go test ./... which exercised repository tests but failed due to unrelated tests in internal/config (TestMigrationWriteErrors expectations), indicating the failure is not caused by this docs-only change.
• Attempted make lint which failed in this environment because no lint target is defined.
• No code changes were made beyond adding the documentation file so all functional behavior remains unchanged by this PR.

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Codex Task