docs: add comprehensive architecture decision records (#359)

[JohnOluB]

PR Description

Closes #359

Summary

This PR introduces a complete Architecture Decision Record (ADR) system for TeachLink, addressing the absence of documented architectural decisions.

What's Added

docs/adr/ — new directory containing:

File | Purpose -- | -- README.md | ADR index, lifecycle states, review process, and relationship to other docs template.md | Reusable template with all required sections and a review checklist 0001-soroban-rust-smart-contracts.md | Why Soroban/Rust was chosen over EVM, CosmWasm, and others 0002-tokenized-learning-rewards.md | Reward architecture: event-based proofs, escrow-gated educator incentives 0003-cross-chain-bridge-design.md | BFT multi-validator bridge design and rejection of trusted relayer model 0004-indexer-technology-selection.md | TypeScript/NestJS indexer rationale 0005-observability-stack.md | Prometheus + Alertmanager + Grafana selection

Acceptance Criteria Met

• ✅ ADR template created — docs/adr/template.md with Context, Decision, Alternatives, Consequences, Implementation Notes, and Review Checklist sections
• ✅ Context documented — each ADR has a dedicated Context section explaining the problem and constraints that forced the decision
• ✅ Consequences tracked — every ADR separates Positive, Negative/Trade-offs, and Neutral consequences
• ✅ Review process established — README.md defines the full lifecycle (Proposed → Accepted → Deprecated/Superseded), merge criteria, and reviewer checklist

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