feat(config): add SPOOLMAN_API_URL with SPOOLMAN_UI_URL alias

[yfridelance]

Summary

Allow SPOOLMAN_BASE_URL (used for UI links) and the URL used for backend
API calls to differ. Adds an optional SPOOLMAN_API_URL override and
introduces SPOOLMAN_UI_URL as a more semantic alias for the existing
SPOOLMAN_BASE_URL.

Fully backward compatible — existing deployments with only
SPOOLMAN_BASE_URL continue to work unchanged.

Problem

Today SPOOLMAN_BASE_URL is used for two distinct concerns:

1. Backend HTTP calls from the OpenSpoolMan container to the Spoolman API
2. UI links rendered into the browser (menu link, spool detail links,
   QR sticker targets)

In a reverse-proxy or split-horizon setup these need different URLs:

• The backend reaches Spoolman via internal Docker DNS, e.g.
  http://spoolman:8000
• The browser must use a public hostname terminated by the reverse
  proxy, e.g. https://spoolman.example.com

Currently users have to choose one. Picking the internal URL gives a
broken "Spoolman" link in the menu pointing to http://spoolman:8000
(unresolvable from a browser). Picking the external URL forces extra
DNS workarounds (extra_hosts, custom dns: entries) so the container
itself can resolve its own public hostname.

Solution

Variable	Required	Purpose	Fallback
SPOOLMAN_BASE_URL (or SPOOLMAN_UI_URL)	yes	UI links shown to the user	—
SPOOLMAN_API_URL	no	Backend API calls	SPOOLMAN_BASE_URL

Resolution order in config.py:

SPOOLMAN_BASE_URL = os.getenv("SPOOLMAN_BASE_URL") or os.getenv("SPOOLMAN_UI_URL")
SPOOLMAN_API_URL  = os.getenv("SPOOLMAN_API_URL")  or SPOOLMAN_BASE_URL
SPOOLMAN_API_V1   = f"{SPOOLMAN_API_URL}/api/v1"

Example: split-horizon deployment

# What users see in the browser (reverse proxy + TLS)
SPOOLMAN_BASE_URL=https://spoolman.example.com

# What the OpenSpoolMan container uses internally
SPOOLMAN_API_URL=http://spoolman:8000

Existing single-URL deployments are unaffected:

SPOOLMAN_BASE_URL=http://spoolman:8000   # used for both, as before

Changes

• config.py — accept SPOOLMAN_UI_URL as alias; add optional
  SPOOLMAN_API_URL with fallback to SPOOLMAN_BASE_URL; replace
  internal references that hit the API to use SPOOLMAN_API_URL /
  SPOOLMAN_API_V1.
• Templates — UI links continue to use SPOOLMAN_BASE_URL (no
  semantic change, just isolated from API resolution).
• config.env.template — document the new optional variable and
  the alias.
• README.md — short note in the configuration section.

Backward compatibility

• No breaking changes.
• All existing env files keep working.
• If neither SPOOLMAN_BASE_URL nor SPOOLMAN_UI_URL is set, behaviour
  is unchanged from today (i.e. unset → existing failure mode).

Test scenarios

• Only SPOOLMAN_BASE_URL set → API calls and UI links use it (current behaviour)
• Only SPOOLMAN_UI_URL set → treated as SPOOLMAN_BASE_URL
• Both SPOOLMAN_BASE_URL and SPOOLMAN_API_URL set → API uses
  SPOOLMAN_API_URL, UI uses SPOOLMAN_BASE_URL
• SPOOLMAN_UI_URL and SPOOLMAN_API_URL set, no SPOOLMAN_BASE_URL
  → UI uses the alias, API uses its own value