add: domain core types

[nccapo]

Here's what was created

• level.go - Level enum (info/warning/error/panic) with JSON marshal/unmarshal
• frame.go - Frame (file, function, line, in_app, source context), SourceLine, ChainedError
• event.go - Event — the full error event struct with all fields from the spec
• group.go - GroupStatus enum + ErrorGroup aggregate
• podcrash.go - CrashType enum + PodCrash struct
• project.go - Project (id, name, dsn_key, created_at)
• domain_test.go - 22 test cases covering every type's JSON roundtrip plus error paths

All three enums (Level, GroupStatus, CrashType) follow the same pattern: int8 under the hood, string JSON representation, Parse* constructor that returns a typed error on unknown values.