feat(TRUEREF-0018): implement embedding provider configuration UI

- Full settings page replacing placeholder with embedding provider selector
- Provider presets: OpenAI, Ollama, Azure OpenAI
- Test Connection button via POST /api/v1/settings/embedding/test
- Warning banner for FTS5-only mode when provider=none
- Local model availability probe (@xenova/transformers)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/routes/api/v1/settings/embedding/test/+server.ts

@@ -0,0 +1,115 @@
+/**
+ * POST /api/v1/settings/embedding/test
+ *
+ * Validates an embedding provider configuration by creating a provider
+ * instance and calling embed(['test']). Returns success with dimensions
+ * or a descriptive error without persisting any changes.
+ */
+
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import {
+	createProviderFromConfig,
+	type EmbeddingConfig
+} from '$lib/server/embeddings/factory';
+import { handleServiceError, InvalidInputError } from '$lib/server/utils/validation';
+
+// ---------------------------------------------------------------------------
+// Validate — reuse the same shape accepted by PUT /settings/embedding
+// ---------------------------------------------------------------------------
+
+function validateConfig(body: unknown): EmbeddingConfig {
+	if (typeof body !== 'object' || body === null) {
+		throw new InvalidInputError('Request body must be a JSON object');
+	}
+
+	const obj = body as Record<string, unknown>;
+
+	const provider = obj.provider;
+	if (provider !== 'openai' && provider !== 'local' && provider !== 'none') {
+		throw new InvalidInputError(
+			`Invalid provider "${String(provider)}". Must be one of: openai, local, none.`
+		);
+	}
+
+	if (provider === 'openai') {
+		const openai = obj.openai as Record<string, unknown> | undefined;
+		if (!openai || typeof openai !== 'object') {
+			throw new InvalidInputError('openai config object is required when provider is "openai"');
+		}
+		if (typeof openai.baseUrl !== 'string' || !openai.baseUrl) {
+			throw new InvalidInputError('openai.baseUrl must be a non-empty string');
+		}
+		if (typeof openai.apiKey !== 'string' || !openai.apiKey) {
+			throw new InvalidInputError('openai.apiKey must be a non-empty string');
+		}
+		if (typeof openai.model !== 'string' || !openai.model) {
+			throw new InvalidInputError('openai.model must be a non-empty string');
+		}
+
+		return {
+			provider: 'openai',
+			openai: {
+				baseUrl: openai.baseUrl as string,
+				apiKey: openai.apiKey as string,
+				model: openai.model as string,
+				dimensions:
+					typeof openai.dimensions === 'number' ? (openai.dimensions as number) : undefined,
+				maxBatchSize:
+					typeof openai.maxBatchSize === 'number' ? (openai.maxBatchSize as number) : undefined
+			}
+		};
+	}
+
+	return { provider: provider as 'local' | 'none' };
+}
+
+// ---------------------------------------------------------------------------
+// POST
+// ---------------------------------------------------------------------------
+
+export const POST: RequestHandler = async ({ request }) => {
+	try {
+		const body = await request.json();
+		const config = validateConfig(body);
+
+		if (config.provider === 'none') {
+			throw new InvalidInputError('Cannot test the "none" provider — no backend is configured.');
+		}
+
+		const provider = createProviderFromConfig(config);
+
+		const available = await provider.isAvailable();
+		if (!available) {
+			return new Response(
+				JSON.stringify({
+					error: `Provider "${config.provider}" is not available. Check your configuration.`
+				}),
+				{ status: 400, headers: { 'Content-Type': 'application/json' } }
+			);
+		}
+
+		// Perform a real embedding call to catch auth / model-name errors.
+		let dimensions: number;
+		try {
+			const results = await provider.embed(['test']);
+			if (results.length === 0) {
+				return new Response(
+					JSON.stringify({ error: 'Provider returned no embeddings for the test input.' }),
+					{ status: 400, headers: { 'Content-Type': 'application/json' } }
+				);
+			}
+			dimensions = results[0].dimensions;
+		} catch (embedErr) {
+			const message = embedErr instanceof Error ? embedErr.message : 'Embedding call failed';
+			return new Response(JSON.stringify({ error: message }), {
+				status: 400,
+				headers: { 'Content-Type': 'application/json' }
+			});
+		}
+
+		return json({ ok: true, dimensions });
+	} catch (err) {
+		return handleServiceError(err);
+	}
+};