[Docs] ARCHITECTURE.md documents wrong backend env var name (STREAMING_CONTRACT_ADDRESS vs STREAM_CONTRACT_ID)

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

docs/ARCHITECTURE.md (Required Environment Variables, ~line 224) lists STREAMING_CONTRACT_ADDRESS as the deployed contract ID var, but the code only ever reads STREAM_CONTRACT_ID (soroban-event-worker.ts:87, sorobanService.ts:5, soroban-indexer.service.ts:30, health.routes.ts:62) - there is no STREAMING_CONTRACT_ADDRESS anywhere in backend/src. Following the doc silently leaves the indexer disabled. Same table also writes STELLAR_NETWORK values as TESTNET/MAINNET uppercase while code uses lowercase testnet.

Acceptance criteria

• Rename STREAMING_CONTRACT_ADDRESS to STREAM_CONTRACT_ID in docs/ARCHITECTURE.md and match backend/.env.example
• Use the lowercase STELLAR_NETWORK casing (testnet/mainnet) consistent with the code
• Cross-check the table against backend/.env.example so var names match what the code reads

Files to touch

• docs/ARCHITECTURE.md
• backend/.env.example
• backend/src/workers/soroban-event-worker.ts

Out of scope

• Frontend NEXT_PUBLIC_* var documentation (separate issue)

[OluRemiFour]

I've reviewed the task and have a clear implementation plan. Looking forward to contributing with a quality PR.