feat: configurable max concurrency for document ingest

[472869879]

Summary

Replace the single-threaded ingest queue with a configurable semaphore allowing up to N documents to be processed in parallel.

Changes

• src/lib/ingest-queue.ts: Global processing boolean replaced with activeCount semaphore. Each concurrent task gets its own AbortController (Map<taskId, AbortController>). cancelTask() targets a specific in-flight task. cancelAllTasks() and pauseQueue() abort all active controllers.
• src/stores/wiki-store.ts + src/lib/project-store.ts: ingestConcurrency added to GeneralConfig. Default: 1 (backward compatible). Persisted to app-state.json with 1-16 validation.
• src/components/settings/sections/general-section.tsx: Number input (1-16) under General settings.
• src/components/settings/settings-types.ts + settings-view.tsx: Draft/save flow for the new field.

Design

• Default = 1: Zero behavior change for existing users.
• Per-task abort: Each concurrent ingest gets an independent AbortController.
• Sweep-on-drain: onQueueDrained only fires when all active tasks complete and no pending tasks remain.
• Stale-context guard: All existing project-switch protections are preserved.

Testing

• 33/33 unit tests pass (ingest-queue.test.ts)
• 12/12 integration tests pass (ingest-queue.integration.test.ts)
• TypeScript typecheck passes
• Vite frontend build succeeds
• Tauri/Rust release build succeeds (Windows MSVC)