Initial scaffold: Flask/SQLite backend, React/Vite PWA frontend

Backend: receipt upload -> LLM vision extraction (OpenAI-compatible,
provider-agnostic), item review/edit, per-group splitting against
Cospend projects/members, highlight+upload via WebDAV, public share
link, bill creation via Cospend's OCS API (verified against real
source, not just doc summaries).

Frontend: capture -> review -> group -> summary flow as an installable
PWA.

install.sh / run.sh (venv + npm, tmux session) instead of Docker, per
the ~/Projects/gain pattern.

backend/app/schema.sql

@@ -0,0 +1,27 @@
+CREATE TABLE IF NOT EXISTS receipts (
+    id TEXT PRIMARY KEY,
+    image_path TEXT NOT NULL,
+    image_width INTEGER,
+    image_height INTEGER,
+    items_json TEXT NOT NULL DEFAULT '[]',
+    store_name TEXT,                    -- extracted from the receipt, editable
+    receipt_date TEXT,                  -- YYYY-MM-DD, extracted; falls back to
+                                         -- today's date if unreadable/unset
+    status TEXT NOT NULL DEFAULT 'extracted',   -- extracted | grouped | done
+    created_at TEXT NOT NULL DEFAULT (datetime('now'))
+);
+
+CREATE TABLE IF NOT EXISTS groups (
+    id TEXT PRIMARY KEY,
+    receipt_id TEXT NOT NULL REFERENCES receipts(id) ON DELETE CASCADE,
+    name TEXT NOT NULL,
+    cospend_project_id TEXT NOT NULL,
+    payer_member_id TEXT NOT NULL,
+    member_ids_json TEXT NOT NULL,   -- json list of cospend member ids (owers)
+    item_ids_json TEXT NOT NULL,     -- json list of item ids from receipts.items_json
+    status TEXT NOT NULL DEFAULT 'pending',   -- pending | submitted | failed
+    share_url TEXT,
+    cospend_bill_id TEXT,
+    error TEXT,
+    created_at TEXT NOT NULL DEFAULT (datetime('now'))
+);