fix: Mainnet deployment guide (Render + UptimeRobot + Neon + external RPC)

[Darktan242]

Summary

Added docs/DEPLOY_MAINNET.md — a full mainnet deployment runbook covering the Render + UptimeRobot + Neon + external RPC stack — and aligned .env.example.

docs/DEPLOY_MAINNET.md (new):

• Full env matrix for testnet + mainnet (STELLAR_NETWORK, HORIZON_URL, RPC_URL, NETWORK_PASSPHRASE, SOROSWAP_FACTORY_ADDRESS, REFLECTOR_CONTRACT_ID, WATCHED_PAIRS, DATABASE_URL, REDIS_URL).
• Documents the mainnet USDC issuer for WATCHED_PAIRS.
• Dedicated "RPC provider" section explaining mainnet has no free Soroban RPC and the endpoint must be a placeholder only.
• "Render keep-alive (UptimeRobot)" section: pings /status every 5 min, notes the 750 instance-hours/month free cap.
• "Database durability (Neon / managed Postgres)" section: warns Render free Postgres is deleted after 90 days, recommends Neon/managed, links the backup runbook (L053 / issue Nightly pg_dump backup workflow + restore runbook #119).
• "Running testnet + mainnet side-by-side" section: separate DB/Redis per network.
• Step-by-step deployment procedure with a placeholder-only mainnet env set.
• No secrets or keys committed anywhere — all credentials are placeholders.

.env.example (aligned):

• Changed RPC_URL_MAINNET from a real endpoint (https://mainnet.sorobanrpc.com) to a placeholder (https://your-provider.example.com/soroban-rpc) with a comment noting mainnet has no free Soroban RPC and real URLs/keys must never be committed.

Changed files

• .env.example
• docs/DEPLOY_MAINNET.md

Test plan

Documentation-only change; no runtime code modified. CI should still pass unchanged (no tests affected). Verification is manual: confirm docs/DEPLOY_MAINNET.md renders correctly, the relative links (../../issues/119, ./DUAL_NETWORK.md, ./architecture.md) resolve, and .env.example still parses (dotenv ignores comments).

This PR was created as a draft by the issue solver bot. It will remain a
draft until repository CI passes.

Closes #120

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