feat(cli): add read-only list and show subcommands

tapir list — table of stored summaries (date, title|id, channel, AI,
fallback), recent-first. tapir show <video-id> — full summary with
highlights and takeaways. DSN + user id from TAPIR_DB_DSN/TAPIR_USER_ID,
never hardcoded. main.go gains a minimal os.Args[1] dispatcher kept flat
so Worker F's auth/run cases union cleanly at merge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/tapir/env.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
+)
+
+// Env var names for the read-only CLI. DSN and user id are never hardcoded — the
+// store holds confidential summaries, so the caller supplies both.
+const (
+	envDSN    = "TAPIR_DB_DSN"
+	envUserID = "TAPIR_USER_ID"
+)
+
+// openStoreFromEnv reads TAPIR_DB_DSN and TAPIR_USER_ID and opens a store. The
+// returned close func releases the pool; callers defer it. Both vars are
+// required: an empty value is a usage error, not a silent default.
+func openStoreFromEnv(ctx context.Context) (s *store.Store, userID string, closeFn func(), err error) {
+	dsn := os.Getenv(envDSN)
+	if dsn == "" {
+		return nil, "", nil, fmt.Errorf("%s is required", envDSN)
+	}
+	userID = os.Getenv(envUserID)
+	if userID == "" {
+		return nil, "", nil, fmt.Errorf("%s is required", envUserID)
+	}
+	s, err = store.New(ctx, dsn)
+	if err != nil {
+		return nil, "", nil, err
+	}
+	return s, userID, s.Close, nil
+}