Centralize runtime configuration

Summary

Centralizes hardcoded runtime configuration into config.php and config.local.php.

Changes

• Adds config.php with documented empty/default config values.
• Adds config.local.php with the currently hardcoded local values copied verbatim.
• Loads config.php first, then overlays config.local.php.
• Adds emalm_config() helper for config lookups.
• Replaces scattered hardcoded reCAPTCHA keys, webhook URLs, CDN paths, site/CDN URLs, and helper service paths with config lookups.
• Leaves database access and conn.php unchanged.

Verification

• Ran PHP lint across all PHP files.
• Ran git diff --check.
• Confirmed extracted secret/path values no longer appear outside config.local.php.