Added support for Postgres polling sources

internal/postgres/postgres.go

@@ -0,0 +1,67 @@
+package postgres
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+	"net/url"
+	"strings"
+	"time"
+
+	_ "github.com/lib/pq"
+)
+
+const initTimeout = 5 * time.Second
+
+var (
+	sqlOpen = sql.Open
+	pingDB  = func(ctx context.Context, db *sql.DB) error { return db.PingContext(ctx) }
+)
+
+// ConnConfig describes the minimal connection settings shared by feedkit's
+// Postgres readers and writers.
+type ConnConfig struct {
+	URI      string
+	Username string
+	Password string
+}
+
+// BuildDSN validates a Postgres URI and injects credentials into it.
+func BuildDSN(cfg ConnConfig) (string, error) {
+	u, err := url.Parse(strings.TrimSpace(cfg.URI))
+	if err != nil {
+		return "", fmt.Errorf("invalid uri: %w", err)
+	}
+	if u.Scheme == "" {
+		return "", fmt.Errorf("invalid uri: missing scheme")
+	}
+	if u.Host == "" {
+		return "", fmt.Errorf("invalid uri: missing host")
+	}
+	u.User = url.UserPassword(cfg.Username, cfg.Password)
+	return u.String(), nil
+}
+
+// Open builds a DSN, opens a database handle, and verifies connectivity with a
+// bounded ping before returning the handle.
+func Open(ctx context.Context, cfg ConnConfig) (*sql.DB, error) {
+	dsn, err := BuildDSN(cfg)
+	if err != nil {
+		return nil, err
+	}
+
+	db, err := sqlOpen("postgres", dsn)
+	if err != nil {
+		return nil, err
+	}
+
+	pingCtx, cancel := context.WithTimeout(ctx, initTimeout)
+	defer cancel()
+
+	if err := pingDB(pingCtx, db); err != nil {
+		_ = db.Close()
+		return nil, err
+	}
+
+	return db, nil
+}