Add configurable sound card channels

[gmisner]

Fixes #82.

This adds a configurable CARD_CHANNELS setting for SVXLink global config generation. It defaults to 2 channels for existing behavior while allowing users with multi-channel sound cards to set the value needed for their hardware.

Changes:

• Adds a Sound Card Channels field to General Settings
• Saves CARD_CHANNELS through the existing settings form update path
• Emits CARD_CHANNELS in the generated SVXLink GLOBAL section, with a fallback of 2 for older databases
• Adds CARD_CHANNELS=2 to bundled install databases, wizard defaults, dev defaults, and sample svxlink.conf
• Adds CARD_CHANNELS to the database update helper for existing installs

Validation:

• git diff --check passed
• php -l settings.php passed
• php -l includes/classes/SVXLink.php passed
• php -l dev_ui/dbupdate.php passed
• wizard/includes/database_defaults.json validates with python3 -m json.tool
• verified bundled install SQLite databases contain CARD_CHANNELS=2

[abcrawford]

This appears to have some conflicts that I will need to look into when I get a moment and have a current system spun up.

Thanks for your contributions.