feat(TRUEREF-0022): complete iteration 0 — worker-thread indexing, parallel jobs, SSE progress

- Move IndexingPipeline.run() into Worker Threads via WorkerPool
- Add dedicated embedding worker thread with single model instance
- Add stage/stageDetail columns to indexing_jobs schema
- Create ProgressBroadcaster for SSE channel management
- Add SSE endpoints: GET /api/v1/jobs/:id/stream, GET /api/v1/jobs/stream
- Replace UI polling with EventSource on repo detail and admin pages
- Add concurrency settings UI and API endpoint
- Build worker entries separately via esbuild

src/routes/settings/+page.svelte

@@ -66,12 +66,23 @@
 	let saveError = $state<string | null>(null);
 	let saveStatusTimer: ReturnType<typeof setTimeout> | null = null;
 
+	let concurrencyInput = $state<number>(0);
+	let concurrencySaving = $state(false);
+	let concurrencySaveStatus = $state<'idle' | 'ok' | 'error'>('idle');
+	let concurrencySaveError = $state<string | null>(null);
+	let concurrencySaveStatusTimer: ReturnType<typeof setTimeout> | null = null;
+
+	$effect(() => {
+		concurrencyInput = data.indexingConcurrency;
+	});
+
 	const currentSettings = $derived(settingsOverride ?? data.settings);
 	const activeProfile = $derived(currentSettings.activeProfile);
 	const activeConfigEntries = $derived(activeProfile?.configEntries ?? []);
 
 	onDestroy(() => {
 		if (saveStatusTimer) clearTimeout(saveStatusTimer);
+		if (concurrencySaveStatusTimer) clearTimeout(concurrencySaveStatusTimer);
 	});
 
 	// ---------------------------------------------------------------------------
@@ -159,6 +170,38 @@
 		void save();
 	}
 
+	async function saveConcurrency() {
+		concurrencySaving = true;
+		concurrencySaveStatus = 'idle';
+		concurrencySaveError = null;
+		try {
+			const res = await fetch('/api/v1/settings/indexing', {
+				method: 'PUT',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ concurrency: concurrencyInput })
+			});
+			if (res.ok) {
+				const updated = await res.json();
+				concurrencyInput = updated.concurrency;
+				concurrencySaveStatus = 'ok';
+				if (concurrencySaveStatusTimer) clearTimeout(concurrencySaveStatusTimer);
+				concurrencySaveStatusTimer = setTimeout(() => {
+					concurrencySaveStatus = 'idle';
+					concurrencySaveStatusTimer = null;
+				}, 3000);
+			} else {
+				const data = await res.json();
+				concurrencySaveStatus = 'error';
+				concurrencySaveError = data.error ?? 'Save failed';
+			}
+		} catch (e) {
+			concurrencySaveStatus = 'error';
+			concurrencySaveError = (e as Error).message;
+		} finally {
+			concurrencySaving = false;
+		}
+	}
+
 	function getOpenAiProfile(settings: EmbeddingSettingsDto): EmbeddingProfileDto | null {
 		return settings.profiles.find((profile) => profile.providerKind === 'openai-compatible') ?? null;
 	}
@@ -482,6 +525,45 @@
 				</div>
 			{/if}
 
+			<!-- Indexing section -->
+			<div class="space-y-3 rounded-lg border border-gray-200 bg-white p-4">
+				<div>
+					<label for="concurrency" class="block text-sm font-medium text-gray-700">
+						Concurrent Workers
+					</label>
+					<p class="mt-0.5 text-xs text-gray-500">
+						Number of parallel indexing workers. Range: 1 to 8.
+					</p>
+				</div>
+
+				<div class="flex items-center gap-3">
+					<input
+						id="concurrency"
+						type="number"
+						min="1"
+						max="8"
+						inputmode="numeric"
+						bind:value={concurrencyInput}
+						disabled={concurrencySaving}
+						class="w-20 rounded-lg border border-gray-300 px-3 py-2 text-sm focus:border-blue-500 focus:outline-none disabled:opacity-50"
+					/>
+					<button
+						type="button"
+						onclick={saveConcurrency}
+						disabled={concurrencySaving}
+						class="rounded-lg bg-blue-600 px-3 py-2 text-sm text-white hover:bg-blue-700 disabled:opacity-50"
+					>
+						{concurrencySaving ? 'Saving…' : 'Save'}
+					</button>
+
+					{#if concurrencySaveStatus === 'ok'}
+						<span class="text-sm text-green-600">✓ Saved</span>
+					{:else if concurrencySaveStatus === 'error'}
+						<span class="text-sm text-red-600">{concurrencySaveError}</span>
+					{/if}
+				</div>
+			</div>
+
 			<!-- Save feedback banners -->
 			{#if saveStatus === 'ok'}
 				<div