feat(TRUEREF-0007): implement pluggable embedding generation and vector storage

Add EmbeddingProvider interface with OpenAI-compatible, local (optional
@xenova/transformers via dynamic import), and Noop (FTS5-only fallback)
implementations. EmbeddingService batches requests and persists Float32Array
blobs to snippet_embeddings. GET/PUT /api/v1/settings/embedding endpoints
read and write embedding config from the settings table.

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

src/lib/server/embeddings/provider.ts

@@ -0,0 +1,47 @@
+/**
+ * EmbeddingProvider interface and NoopEmbeddingProvider.
+ *
+ * The Noop provider returns null embeddings and enables FTS5-only mode
+ * when no embedding backend is configured.
+ */
+
+export interface EmbeddingVector {
+	values: Float32Array;
+	dimensions: number;
+	model: string;
+}
+
+export interface EmbeddingProvider {
+	readonly name: string;
+	readonly dimensions: number;
+	readonly model: string;
+
+	embed(texts: string[]): Promise<EmbeddingVector[]>;
+	isAvailable(): Promise<boolean>;
+}
+
+export class EmbeddingError extends Error {
+	constructor(message: string) {
+		super(message);
+		this.name = 'EmbeddingError';
+	}
+}
+
+/**
+ * NoopEmbeddingProvider — always returns empty results.
+ * Used as the default when no provider is configured; the system
+ * falls back gracefully to FTS5-only search.
+ */
+export class NoopEmbeddingProvider implements EmbeddingProvider {
+	readonly name = 'noop';
+	readonly dimensions = 0;
+	readonly model = 'none';
+
+	async embed(_texts: string[]): Promise<EmbeddingVector[]> {
+		return [];
+	}
+
+	async isAvailable(): Promise<boolean> {
+		return false;
+	}
+}