fix: set company placeholder to UNKNOWN and normalize priority to uppercase

[Pcmhacker-piro]

📝 Use this PR Body:

Closes #686

## Problem
`build_slack_payload()` used `"Unknown"` (mixed case) as the fallback
for the company field, but tests expect `"UNKNOWN"` (uppercase).
Priority values were also not normalized, causing inconsistent casing.

## Fix
- Company fallback changed to `"UNKNOWN"` (uppercase)
- Priority normalized via `.upper()` to handle any input casing

## File Changed
- `backend/services/slack_notifier.py`

## Tests
✅ 338 passed, 1 skipped — all existing tests pass

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Why this matters:

• Title tells the maintainer exactly what was fixed at a glance
• "Closes #686" in the body automatically links and closes the issue when merged
• Test results show your change doesn't break anything

[Pcmhacker-piro]

@SdSarthak

Hi, the checks have passed. Could you please review and approve the pending workflows when you have a chance? Thank you!