Centralize SkyPortal config

[MenesesCarlos29]

The current SkyPortal scripts duplicate settings such as API base URL, output paths, request defaults, and inventory parameters. This task is to move shared runtime settings into a versioned SkyPortal config file, while keeping secrets in .env.

The goal is to support reusable inventory profiles for
02_fetch_source_inventory.py and
01_audit_endpoint_availability.py.

Scope:

• create extraction/skyportal.yaml with shared settings such as:
  • base_url
  • output directories
  • request defaults (timeout, sleep, max_retries, num_per_page)
  • reusable inventory profiles such as recent_500, has_spectrum,
    has_followup, classified, redshift, gcn, ep
• add a reusable Python config loader that:
  • reads the YAML file
  • validates required sections
  • exposes config values in a format the scripts can use
• update 02_fetch_source_inventory.py so it can:
  • read defaults from config
  • run inventory profiles by name
  • still allow CLI arguments to override config values
• update 01_audit_endpoint_availability.py so it can:
  • read shared defaults from config
  • reuse config values such as base_url, output paths, timeout, and sleep
• update documentation to explain:
  • what goes in skyportal.yaml
  • what stays in .env
  • how to run scripts with profiles
  • how CLI overrides work

Out of scope:

• moving API tokens or secrets to YAML
• moving the endpoint specification list out of Python

Acceptance criteria:

• extraction/skyportal.yaml exists and is documented
• 02_fetch_source_inventory.py can execute inventory profiles by name
• 01_audit_endpoint_availability.py can read shared defaults from config
• CLI arguments override config values when both are provided
• .env remains the source of secrets
• related documentation is updated