Show notes from similar earlier incidents

Each incident gets a signature: the alert name plus the group labels that
say what is broken, minus the ones that only say where it ran (instance,
pod, container, ...). GET /api/incidents/{id}/similar returns resolved
incidents in the same team with the same signature that have notes.

Notes can be marked as the resolution note, "what fixed it", either with a
resolution field on resolve or pinned on a note. Those lead the similar
list, show on the incident page as "Seen before", and the triggered
notification carries the latest one.

Claude-Session: https://claude.ai/code/session_01MMados3BD1oSjevHxbmVqU

internal/models/incident.go

@@ -79,3 +79,15 @@ type IncidentEvent struct {
 	Detail     *string   `json:"detail,omitempty"`
 	CreatedAt  time.Time `json:"created_at"`
 }
+
+// SimilarIncident is an earlier, resolved incident with the same signature as
+// the one being looked at. ResolutionNotes are the "what fixed it" notes;
+// NoteCount counts the plain working notes, which live on the timeline.
+type SimilarIncident struct {
+	ID              int64           `json:"id"`
+	Title           string          `json:"title"`
+	TriggeredAt     time.Time       `json:"triggered_at"`
+	ResolvedAt      time.Time       `json:"resolved_at"`
+	NoteCount       int             `json:"note_count"`
+	ResolutionNotes []IncidentEvent `json:"resolution_notes"`
+}