Allow schedule import to create a festival/edition on the fly

[chiptus]

Problem

Today the schedule import wizard (Admin → Schedule import) only runs inside an existing edition — you have to create the festival and edition first, then navigate into it, before you can import a lineup CSV.

For a brand-new event (e.g. seeding a lineup-phase announcement before any schedule details exist), this means a manual "create festival + create edition" detour before the actual import can start.

Proposal

Support kicking off an import from the main festival list view, where the target festival/edition doesn't exist yet:

• Let the user pick "new festival" (or "new edition" under an existing festival) as part of the import flow, instead of requiring it to already exist.
• On commit, create the festival/edition alongside the diffed sets, in the same transaction as the rest of the import.

Context

Came up while building the CSV import for Desert Base 2026 lineup phase A (desert-base/2026/desert-base-2026-lineup-phase1-import.csv) — that import only has artist names, no schedule/stage/date yet, and had to be pointed at a manually pre-created edition.

See docs/schedule-import.md for how the current pipeline works (parse → diff → review → commit).

Use /grill-with-docs to work out the UX and transaction-boundary details before implementation.