feat: print API - exact single image, sheet PDF, sheet presets

POST /api/print/single (image at an exact size, dpi headers incl. a real-dpi
warning), GET /api/print/single as a mm-to-px calculator, POST /api/print/sheet
(multi-image sheet as PDF, limits on cells/copies/pages) and print_presets for
reusable sheet setups.

migrations/013_print_presets.sql

@@ -0,0 +1,14 @@
+-- Druckbogen-Vorlagen („Kita-Satz Felix", „8× Passbild") -------------------
+CREATE TABLE IF NOT EXISTS print_presets (
+  id         uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  name       text NOT NULL,
+  created_by uuid REFERENCES users(id) ON DELETE SET NULL,
+  config     jsonb NOT NULL,
+  created_at timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX IF NOT EXISTS print_presets_name_idx ON print_presets(name);
+
+-- Telegram: Zustand „wartet auf Druck-Auswahl" -----------------------------
+ALTER TABLE telegram_drafts DROP CONSTRAINT IF EXISTS telegram_drafts_status_check;
+ALTER TABLE telegram_drafts ADD CONSTRAINT telegram_drafts_status_check
+  CHECK (status IN ('collecting','awaiting_recipe','awaiting_compose_text','awaiting_print','dispatched','discarded'));