fix: trim whitespace from domain names before saving hosts

[addielaruee]

Why

Fixes #5654.

When a domain/host is entered with a leading or trailing space (easy to do via copy‑paste), NPM stores the space verbatim. The generated nginx config then contains an invalid server_name/upstream, so the host saves but never loads — the user sees a misleading 502 that looks like an upstream problem rather than bad input.

Fix

The proxy_host, dead_host and redirection_host models each sorted domain_names in their $beforeInsert/$beforeUpdate hooks but never trimmed them, and certificate.js carried its own near‑identical (also untrimmed) cleanDomainNames.

This adds a single shared cleanDomainNames helper in backend/lib/helpers.js that:

• drops null/undefined entries,
• trims surrounding whitespace,
• removes any entry left empty after trimming,
• sorts,

and uses it across all four models, replacing the ad‑hoc per‑model .sort() and removing the duplicated copy in certificate.js. Trimming now applies consistently to every host type (and certificates) at the persistence layer.

Behaviour is a superset of the previous logic (which already filtered null and sorted), so existing well‑formed input is unaffected.

Verification

• biome lint passes on all five changed files.

• Unit-style checks of the helper:

  Input	Output
  [" example.com ", "b.com"]	["b.com", "example.com"]
  [" spaced.test "]	["spaced.test"]
  ["z.com", "a.com", ""]	["a.com", "z.com"]
  ["keep.com", null, " ", " x.io"]	["keep.com", "x.io"]
  undefined	[]

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• API changes
• Performance improvement
• Test addition or update

AI Usage

• AI was used to write this
• AI was used to review this