fix(cli): implement CHARON_PRICE_MAX_AGE_SECS env override (closes #413)

[obchain]

Summary

The docstring on `BotConfig::chainlink_max_age_secs` and the operator-facing comment in `scripts/anvil_fork.sh` both reference `CHARON_PRICE_MAX_AGE_SECS` as a documented escape hatch for the Chainlink freshness gate. The env var was never implemented — `grep -rn CHARON_PRICE_MAX_AGE_SECS crates/` returned only the docstring. Both `PriceCache::with_per_symbol_max_age` call sites in charon-cli (listen + replay) hardcoded `DEFAULT_MAX_AGE`. Operators following the documented recovery flow on a stale fork hit `feed 'BNB' missing or stale ... max_age 3600 s` regardless of the env var.

Fix

• Add `resolve_default_price_max_age()` helper in charon-cli that reads `CHARON_PRICE_MAX_AGE_SECS` (u64 seconds), logs once on success, warns on parse failure, and falls back to `DEFAULT_MAX_AGE`.
• Use the helper at both `PriceCache::with_per_symbol_max_age` call sites (`listen` path and `replay` path).
• Per-symbol overrides in `[chainlink_max_age_secs.]` continue to win over the env-resolved global default.
• Update the `BotConfig` docstring to describe the actual behaviour so the docs stop misrepresenting the wiring.

Test plan

• `cargo check --bin charon` clean (verified locally).
• `CHARON_PRICE_MAX_AGE_SECS=86400 charon listen` accepts a feed that was rejected at the 600s default.
• `CHARON_PRICE_MAX_AGE_SECS=abc charon listen` warns and falls back without aborting startup.
• `CHARON_PRICE_MAX_AGE_SECS` unset behaves identically to current main.

Do NOT merge until #412 + #414 land and the local fork-test passes end-to-end.