feat: add PostgreSQL persistence layer with Neon DB support

internal/teachingassets/store_pg.go

@@ -0,0 +1,53 @@
+package teachingassets
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+type PostgresStore struct {
+	pool *pgxpool.Pool
+}
+
+func NewPostgresStore(pool *pgxpool.Pool) *PostgresStore {
+	return &PostgresStore{pool: pool}
+}
+
+func toJSON(v any) string {
+	b, _ := json.Marshal(v)
+	return string(b)
+}
+
+func (s *PostgresStore) SavePrompt(prompt PromptCandidate) (PromptCandidate, error) {
+	_, err := s.pool.Exec(context.Background(),
+		`INSERT INTO teaching_asset_prompts (id, concept_id, asset_type, prompt, model_key, review_state, requires_model_id_verification, source_evidence, created_at)
+		 VALUES ($1, $2, $3, $4, $5, $6, $7, $8::jsonb, $9)`,
+		prompt.ID, prompt.Concept.ID, string(prompt.AssetType), prompt.Prompt,
+		prompt.ModelKey, string(prompt.ReviewState), prompt.RequiresModelIDVerification,
+		toJSON(prompt.SourceEvidence), prompt.CreatedAt,
+	)
+	if err != nil {
+		return PromptCandidate{}, fmt.Errorf("insert prompt: %w", err)
+	}
+	return prompt, nil
+}
+
+func (s *PostgresStore) Snapshot() Snapshot {
+	rows, _ := s.pool.Query(context.Background(),
+		`SELECT id, concept_id, asset_type, prompt, model_key, review_state, requires_model_id_verification, source_evidence, created_at FROM teaching_asset_prompts`)
+	defer rows.Close()
+
+	var prompts []PromptCandidate
+	for rows.Next() {
+		var p PromptCandidate
+		var evidenceJSON string
+		rows.Scan(&p.ID, &p.Concept.ID, &p.AssetType, &p.Prompt, &p.ModelKey,
+			&p.ReviewState, &p.RequiresModelIDVerification, &evidenceJSON, &p.CreatedAt)
+		json.Unmarshal([]byte(evidenceJSON), &p.SourceEvidence)
+		prompts = append(prompts, p)
+	}
+	return Snapshot{Prompts: prompts}
+}