feat: SNMP trap provider (v1/v2c/v3 with USM auth/priv)

[ykd007]

Closes #2112\n\nBeen meaning to get SNMP trap ingestion working for a while. This adds a new provider that listens for traps over UDP and turns them into Keep alerts — covering v1, v2c, and v3 with USM auth/priv.\n\nWhat's in here:\n\n- UDP trap listener that runs as a background thread, feeds a bounded queue\n- SNMPv1 and v2c: community string filtering, enterprise OID and varbind extraction\n- SNMPv3: USM with SHA/MD5 auth and AES/AES256/DES encryption config\n- Well-known trap OIDs mapped to human names + severity (linkDown, linkUp, coldStart, authFailure, etc.)\n- Raw BER decoder as fallback if pysnmp isn't available\n- Mock alerts for UI preview\n\nTested manually with against a local listener. The v3 path requires pysnmp; if it's not installed the BER fallback handles v1/v2c cleanly.\n\nPart of the Algora bounty on this issue.

[ykd007]

Pushed a fix commit addressing two correctness issues:

1. BER v2c varbind parsing was a stub — was returning a placeholder string instead of actually walking the VarBindList. Now fully decodes each OID/value pair, handles OCTET STRING (UTF-8), INTEGER, and raw hex for other types.

2. SNMPv3 decode path was broken — protoModules[protoVersion3] is not a valid key in pysnmp; the v3 path always fell through to BER. Fixed to use pyasn1 BER decoder with rfc3412.Message and rfc3414.ScopedPDU to extract varbinds directly from the scoped PDU.

Both the pysnmp and BER fallback paths now correctly return populated AlertDtos.

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[ykd007]

I have read the CLA Document and I hereby sign the CLA

[shahargl]

N/A