Rename mt5api env vars to MT5API_

[dceoy]

Summary

• rename the app environment variable prefix from API_ to MT5API_
• update configuration lookups, CLI/config tests, and router-prefix validation messaging
• refresh README, docs, and quickstart examples to use the new variable names

Testing

• /Users/dceoy/util/mt5api/.agents/skills/local-qa/scripts/qa.sh
• uv run mkdocs build

[claude]

Clean, well-executed rename. The code changes are consistent — all ENV_API_* constants, config lookups, tests, and documentation within the PR correctly use the new MT5API_ prefix. No stale references remain in any .py file or in the docs/specs touched by this PR.

Positive notes:

• Good separation between ENV_MT5API_* (user-facing env var names) and DEFAULT_API_* (internal defaults) — no need to rename the latter.
• Tests import constants symbolically rather than hardcoding strings, which made this rename safe and verifiable.
• The error message update ("Invalid MT5API_ROUTER_PREFIX") and its corresponding test assertion are in sync.

One issue flagged below as inline comments.

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