ref(server): Remove unnecessary old code for splitting envelopes

[Dav1dde]

• Removes split_envelope logic
• Everything which is associated with split_envelope and ProcessingGroup
• Moves processing into separate RelayProcessor with effectively the same processing logic as before, just easier to understand (hopefully)
• Makes dropping invalid data explicit (FormData)
• Raw security reports created multiple errors (and envelopes) from multiple items, these should've been individual envelopes from the beginning (they are now)
• COGS are tracked slightly differently, I don't expect a noticeable change, will monitor.
• Does not do any additional cleanup which is now possible, e.g. fn process_envelope can be merged into fn process

[cursor]

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