feat: additional findings processor

[lcampbell2]

Adds a new scanners/findings-processor Go service that consumes finding events from NATS JetStream and upserts normalized documents into ArangoDB additionalFindings.

• Introduces a standalone service scaffold (cmd/service, internal/*, go.mod, Dockerfile, Makefile, cloudbuild, env template, README).
• Implements config/env loading, structured logging, and graceful shutdown.
• Subscribes to scans.findings.* (configurable), validates event payloads, and applies explicit Ack/Nak/Term handling.
• Adds finding normalization + deterministic key generation, then upsert logic that creates new docs or updates lastSeen and occurrence count for existing docs.
• Includes broad unit test coverage across config, model validation/mapping, DB upsert flow, and runner/event behavior.

[lcampbell2]

/gcbrun

[lcampbell2]

/gcbrun(84c5a2c)