fix(prometheus): tolerate malformed exporter env

[yyswhsccc]

Problem

prometheus_exporter.py parsed PROMETHEUS_EXPORTER_PORT and SCRAPE_INTERVAL directly at import time. A malformed deployment env value raises ValueError before the exporter can fall back or start.

Impact

A single bad numeric env value can crash the RustChain Prometheus exporter during startup, removing metrics visibility until the environment is corrected.

Fix

Add a small local _safe_int_env helper that logs malformed numeric env values and falls back to the existing defaults. Valid numeric env overrides are preserved.

Tests

• uv run --no-project --with pytest --with flask --with requests python -B -m pytest -q tests/test_prometheus_exporter_security.py -> 5 passed
• python3 -m py_compile prometheus_exporter.py tests/test_prometheus_exporter_security.py -> passed
• git diff --check -> passed

Boundaries

Related to the general bug bounty surface (#305). This PR does not change payout amounts, wallet crediting, admin secrets, or production wallet behavior.

wallet: RTC47bc28896a1a4bf240d1fd780f4559b242bcd945

[yyswhsccc]

@Scottcjn Please take a look when you have a chance. This is a small startup hardening fix for the Prometheus exporter: malformed numeric env values now fall back to existing defaults instead of crashing import/startup.

Validation:

• uv run --no-project --with pytest --with flask --with requests python -B -m pytest -q tests/test_prometheus_exporter_security.py -> 5 passed
• python3 -m py_compile prometheus_exporter.py tests/test_prometheus_exporter_security.py -> passed
• git diff --check -> passed

[jaxint]

✅ Code reviewed - implementation verified.

[jaxint]

✅ Code reviewed - implementation verified. Good work on the changes.

[jaxint]

Good addition! This will be useful for the project.

Reviewed for Bounty #71
Wallet: AhqbFaPBPLMMiaLDzA9WhQcyvv4hMxiteLhPk3NhG1iG

[jaxint]

✅ Code reviewed - implementation verified. Per bounty #71.