Add lowerCamelCase JSON tags to canonical model types (observation, forecast,
alert) to stabilize the emitted wire format and make payload structure explicit
for downstream sinks.
Introduce internal/model/doc.go to document these structs as versioned,
schema-governed payloads and clarify compatibility expectations (additive
changes preferred; breaking changes require schema bumps).
No functional behavior changes; this formalizes the canonical output contract
ahead of additional sinks and consumers.
