feat: Add SvelteKit web app with scan sessions and import queue

Replaces the Flask/Alpine web app with a SvelteKit 2 + Svelte 5 rewrite
under web/, built on adapter-node and Tailwind v4. Same shape as the
reference b4l budget app — no auth, stateless pass-through to InvenTree.

New "scan session" flow groups mass scans into a session with live
counters (scanned / succeeded / pending / failed). Unknown parts in
import mode are fed to a worker pool that spawns inventree-part-import
(IMPORT_CONCURRENCY, default 3, with 3-retry). Anything that can't be
resolved automatically — parse errors, missing qty, invalid location,
API errors, or imports that exhaust retries — drops into a Failures
panel with a per-item Fix dialog (edit fields / search existing part /
retry import). CSV export on the failure list.

Layout is two-column on lg+: scanner + activity on the left, pending
imports + failures on the right. Light-theme default. SSE on
/api/events streams session and import events to the client.

Barcode parser ported from the Python/JS versions and hardened for
Digi-Key MH10.8.2 barcodes both with and without GS separators (old
parser greedy-matched Q's digits and read "Q6" + "11ZPICK" as 611).
Import worker also now treats a subprocess failure followed by a
successful findPart as a success, so partial imports (part created but
a duplicate parameter trips the DB constraint) no longer land in the
Failures panel.

Deploy artifacts: systemd unit, nginx example (SSE-friendly), and a
step-by-step deploy/README. Requires inventree-part-import >= 1.9.2 on
the server for InvenTree 1.x API compatibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/README.md

@@ -0,0 +1,73 @@
+# InvenTree Stock Tool — Web (SvelteKit)
+
+SvelteKit rewrite of the Flask/Alpine web app.
+
+## Stack
+
+- SvelteKit 2 + Svelte 5 (runes) + adapter-node
+- Tailwind CSS v4
+- In-process scan-session store (no database)
+- Server-Sent Events for live session/import updates
+- Shell-out to `inventree-part-import` for unknown-part creation
+
+## Dev
+
+```bash
+cd web
+npm install
+cp .env.example .env
+# edit .env to point at your InvenTree server
+npm run dev
+```
+
+Open http://localhost:5173.
+
+## Build
+
+```bash
+npm run build
+node build
+```
+
+## Feature: Scan Session
+
+- A session groups a run of scans. Every scan goes through `/api/session/scan`
+  and is recorded in the session's stats.
+- Unknown parts in "Add Stock" mode are enqueued for `inventree-part-import`;
+  workers run in parallel (tunable via `IMPORT_CONCURRENCY`, default 3).
+- Failures (unknown parts outside import mode, missing qty, invalid location,
+  API errors, and imports that exhaust 3 retries) land in the Failures panel.
+- Each failure has a **Fix** dialog — override the part code / quantity /
+  location, or search for an existing part and link it instead. Retry feeds
+  back through the same code path.
+- Export CSV of failures for offline follow-up.
+
+## Layout
+
+```
+web/
+├── src/
+│   ├── app.css, app.html, app.d.ts
+│   ├── lib/
+│   │   ├── barcode.ts          shared parse_scan + commands
+│   │   ├── client.ts           fetch wrappers for API routes
+│   │   ├── types.ts            shared types
+│   │   ├── server/             ← imported only from +server.ts / hooks
+│   │   │   ├── env.ts          typed env config
+│   │   │   ├── inventree.ts    8 API helpers
+│   │   │   ├── events.ts       SSE pubsub
+│   │   │   ├── importQueue.ts  N-worker pool → inventree-part-import
+│   │   │   └── sessions.ts     in-memory scan sessions
+│   │   ├── stores/app.svelte.ts
+│   │   └── components/…
+│   └── routes/
+│       ├── +layout.svelte, +page.svelte
+│       └── api/
+│           ├── config, locations, proxy/image, part/search, events
+│           └── session/{start,end,mode,location,scan,retry,failures}
+└── deploy/      systemd unit, nginx example, deploy README
+```
+
+## Deployment
+
+See [`deploy/README.md`](deploy/README.md).