feat: Treat lower priority Usenet providers as backups

[banhathome]

📌 Description

• Treat lower priority Usenet providers as backups. See (Usenet) Lower priority providers should behave as backups #299 for more details.

---

Target Branch Check (IMPORTANT)

• I confirm this PR is targeting the correct branch

Expected target:

• beta (for features)

---

Changes Made

• Rework ExecuteWithFailover() and getConnectionFromProvider() in internal/nntp/client.go to always attempt to find and execute using a connection from a higher priority (lower index) providers first.
• Remove getAnyAvailableConnection()

---

Testing

(Will do more thorough testing in a bit)

• Tested locally

Steps:

---

Risks / Notes

• Currently, the connection pool shows as the sum of the max connections of each Usenet provider. This UI probably needs to be changed because some users will think it's a bug that Decypharr rarely uses more than X connections (X = max connections for their highest priority provider).
• Needs to be reviewed to ensure that code does not perform an infinite loop of trying to execute with the same provider despite clear errors that the provider is not usable for this task.
  • Retry behavior should prevent this from occurring

---

Screenshots (if applicable)

---

Checklist

(Will do more thorough testing in a bit)

• Code builds successfully
• No console/log errors
• Reviewed my own code
• Target branch is correct