[FEATURE]: Add env var to listen on specific interface

[ScrumpyJack]

Is your feature request related to a problem? Please describe.
Listening on all interfaces can be problematic, especially when running the binary on the host OS (not containerised)
Please allow the option to listen on specific interface (with a LISTEN env var - don't use HOST, that's a global env var)

[coderabbitai]

A summary of the changes CodeRabbit can apply:

• Add a new LISTEN environment variable (default "0.0.0.0") by adding a Listen field and default in internal/config/config.go, passing os.Getenv("LISTEN") from cmd/capture/main.go into NewConfig, replacing the hardcoded bind in internal/server/server.go to use config.Listen, and update README.md plus all example config files (Docker Compose, systemd) to document and demonstrate the variable.

• Add a new LISTEN configuration to allow binding to a configurable network interface (default 0.0.0.0) by updating internal/config/config.go (add Listen field, defaultListen, NewConfig signature and defaults, Default()), wiring os.Getenv("LISTEN") into cmd/capture/main.go when creating the config, changing server bind from hardcoded "0.0.0.0:PORT" to config.Listen:config.Port in internal/server/server.go, and documenting the variable in README.md, deployment/reverse-proxy-compose/caddy.compose.yml, and docs/systemd/capture.service.

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