feat: add Mezmo (LogDNA) webhook provider

[selenaalpha77-sketch]

Closes #1838

Summary

Adds a Mezmo (formerly LogDNA) provider that receives log-based alert webhooks from Mezmo's Views/Alerts system. Follows Keep's webhook provider pattern (same as Checkmk, SNMP, Wazuh).

Files added:

• keep/providers/mezmo_provider/mezmo_provider.py — webhook provider, _format_alert maps Mezmo payload to AlertDto
• keep/providers/mezmo_provider/alerts_mock.py — 3 sample payloads (error, critical, warning)
• keep/providers/mezmo_provider/__init__.py
• docs/providers/documentation/mezmo-provider.mdx — setup guide with payload reference

Severity mapping: Mezmo log levels (critical/error/warning/info/debug) → Keep severity

Timestamp handling: accepts both ISO-8601 strings and epoch-millisecond integers

No external dependencies required.

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