feat: Add SNMP Provider (v1/v2c/v3 trap receiver)

[justintadair-debug]

SNMP Provider

Closes #2112

/attempt #2112
/claim #2112

Adds a webhook-based SNMP trap receiver that converts SNMP traps into Keep alerts.

Features

• SNMPv1, v2c, and v3 support
• Well-known OID auto-mapping (linkDown, linkUp, coldStart, warmStart, authenticationFailure, egpNeighborLoss) with appropriate severities
• Fingerprint-based deduplication on [trap_oid, agent_address]
• Follows existing provider patterns (webhook receiver like Checkmk)
• No heavy pysnmp dependency — receives pre-parsed trap JSON from snmptrapd
• Mock alert data included for testing

Files

• keep/providers/snmp_provider/init.py
• keep/providers/snmp_provider/snmp_provider.py
• keep/providers/snmp_provider/alerts_mock.py

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