feat: generate transcript and event-driven particle processing

This generates the transcript and shows the caption experience on the
client side for media particles. It also simplifies other side effects
that we must perform such as updating the `last_child_created_at` field
for stream and container particles.

go/internal/particle/processing_repository.go

@@ -0,0 +1,37 @@
+package particle
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+type ProcessingRepository interface {
+	IsProcessed(ctx context.Context, particleID string) (bool, error)
+	MarkProcessed(ctx context.Context, particleID string) error
+}
+
+type processingRepositoryImpl struct {
+	pool *pgxpool.Pool
+}
+
+func NewProcessingRepository(pool *pgxpool.Pool) ProcessingRepository {
+	return &processingRepositoryImpl{pool: pool}
+}
+
+func (r *processingRepositoryImpl) IsProcessed(ctx context.Context, particleID string) (bool, error) {
+	var exists bool
+	err := r.pool.QueryRow(ctx,
+		`SELECT EXISTS(SELECT 1 FROM processed_particles WHERE particle_id = $1)`,
+		particleID,
+	).Scan(&exists)
+	return exists, err
+}
+
+func (r *processingRepositoryImpl) MarkProcessed(ctx context.Context, particleID string) error {
+	_, err := r.pool.Exec(ctx,
+		`INSERT INTO processed_particles (particle_id) VALUES ($1) ON CONFLICT (particle_id) DO NOTHING`,
+		particleID,
+	)
+	return err
+}