docs: fix README — correct project name, broken code blocks, placeholder text, and API accuracy

[Uchechukwu-Ekezie]

What changed

• Project name corrected throughout — was "Stellar Portfolio Rebalancer" everywhere; now correctly reads "SentientFi"
• Broken code blocks fixed — 12+ instances of bash# comment (language tag fused to code) replaced with properly fenced blocks
• env blocks fixed — envCONTRACT_ADDRESS=... style replaced with proper fenced env blocks
• Missing project title added — # SentientFi heading was absent; README started mid-sentence
• Clone URL corrected — was https://github.com/your-username/stellar-portfolio-rebalancer.git (placeholder + wrong name)
• Placeholder text removed — [Hackathon Name] replaced with descriptive wording
• API reference corrected — removed claim that /api/v1/* is the canonical namespace (v1Router is not currently mounted; /api/* is what works)
• Price feeds clarified — noted CoinGecko is current, Reflector backend integration is planned (Phase 2)
• Architecture table added — tech stack presented in a readable table
• ADMIN_PUBLIC_KEYS documented — was missing from configuration examples, causing admin routes to 503 on fresh installs
• Docker deployment section updated — structure and commands kept accurate
• Roadmap updated — Phase 2 now lists HMAC webhook signatures and Reflector integration as tracked items

Why

The README was displaying incorrect/placeholder content that doesn't match the actual project, making it hard for new contributors to set up the project correctly.

How to test

git clone https://github.com/grantFoxin/SentientFi.git
cd SentientFi
# Follow Quick Start steps in the updated README

Verify the code blocks render correctly on GitHub's README preview.