feat: add JSON serialization, urgency field, and snake_case API contract

Fix latent API bug where multi-word fields (RecurrenceDuration, ParentUUID,
CreatedAt) serialized as PascalCase, breaking the frontend. Add explicit
snake_case json tags and custom MarshalJSON/UnmarshalJSON on Task, Status,
and APIKey to emit unix timestamps and string status codes.

Add Urgency float64 as a derived field on Task, populated via
PopulateUrgency helper in all handlers before serialization. The report
engine's sortByUrgency now also retains the computed score.

Frontend updated with urgency type, color-coded badge in TaskItem, and
mock data values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/auth.go

@@ -3,6 +3,7 @@ package engine
 import (
 	"crypto/rand"
 	"encoding/base64"
+	"encoding/json"
 	"fmt"
 	"time"
 
@@ -11,12 +12,69 @@ import (
 
 // APIKey represents an API key in the database
 type APIKey struct {
-	ID        int
-	Name      string
-	UserID    int
-	CreatedAt time.Time
-	LastUsed  *time.Time
-	Revoked   bool
+	ID        int        `json:"id"`
+	Name      string     `json:"name"`
+	UserID    int        `json:"user_id"`
+	CreatedAt time.Time  `json:"created_at"`
+	LastUsed  *time.Time `json:"last_used,omitempty"`
+	Revoked   bool       `json:"revoked"`
+}
+
+// MarshalJSON emits APIKey with unix timestamps.
+func (k APIKey) MarshalJSON() ([]byte, error) {
+	type keyJSON struct {
+		ID        int    `json:"id"`
+		Name      string `json:"name"`
+		UserID    int    `json:"user_id"`
+		CreatedAt int64  `json:"created_at"`
+		LastUsed  *int64 `json:"last_used,omitempty"`
+		Revoked   bool   `json:"revoked"`
+	}
+
+	var lastUsed *int64
+	if k.LastUsed != nil {
+		v := k.LastUsed.Unix()
+		lastUsed = &v
+	}
+
+	return json.Marshal(keyJSON{
+		ID:        k.ID,
+		Name:      k.Name,
+		UserID:    k.UserID,
+		CreatedAt: k.CreatedAt.Unix(),
+		LastUsed:  lastUsed,
+		Revoked:   k.Revoked,
+	})
+}
+
+// UnmarshalJSON parses APIKey from JSON with unix timestamps.
+func (k *APIKey) UnmarshalJSON(data []byte) error {
+	type keyJSON struct {
+		ID        int    `json:"id"`
+		Name      string `json:"name"`
+		UserID    int    `json:"user_id"`
+		CreatedAt int64  `json:"created_at"`
+		LastUsed  *int64 `json:"last_used,omitempty"`
+		Revoked   bool   `json:"revoked"`
+	}
+
+	var raw keyJSON
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return err
+	}
+
+	k.ID = raw.ID
+	k.Name = raw.Name
+	k.UserID = raw.UserID
+	k.CreatedAt = time.Unix(raw.CreatedAt, 0)
+	k.Revoked = raw.Revoked
+
+	if raw.LastUsed != nil {
+		t := time.Unix(*raw.LastUsed, 0)
+		k.LastUsed = &t
+	}
+
+	return nil
 }
 
 // GenerateAPIKey creates a new API key for the given name