model: add explicit JSON tags and document canonical payload contract

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.

internal/model/alert.go

@@ -1,23 +1,24 @@
+// FILE: internal/model/alert.go
 package model
 
 import "time"
 
 // Placeholder for NWS alerts (GeoJSON feature properties are rich).
 type WeatherAlert struct {
-	ID string
+	ID string `json:"id"`
 
-	Event       string
-	Headline    string
-	Description string
-	Instruction string
+	Event       string `json:"event,omitempty"`
+	Headline    string `json:"headline,omitempty"`
+	Description string `json:"description,omitempty"`
+	Instruction string `json:"instruction,omitempty"`
 
-	Severity  string
-	Urgency   string
-	Certainty string
+	Severity  string `json:"severity,omitempty"`
+	Urgency   string `json:"urgency,omitempty"`
+	Certainty string `json:"certainty,omitempty"`
 
-	Sent      *time.Time
-	Effective *time.Time
-	Expires   *time.Time
+	Sent      *time.Time `json:"sent,omitempty"`
+	Effective *time.Time `json:"effective,omitempty"`
+	Expires   *time.Time `json:"expires,omitempty"`
 
-	Areas []string
+	Areas []string `json:"areas,omitempty"`
 }