feat(TRUEREF-0020): add embedding profiles, default local embeddings, and version-scoped semantic retrieval

- Add embedding_profiles table with provider registry pattern
- Install @xenova/transformers as runtime dependency
- Update snippet_embeddings with composite PK (snippet_id, profile_id)
- Seed default local profile using Xenova/all-MiniLM-L6-v2
- Add provider registry (local-transformers, openai-compatible)
- Update EmbeddingService to persist and retrieve by profileId
- Add version-scoped VectorSearch with optional versionId filtering
- Add searchMode (auto|keyword|semantic|hybrid) to HybridSearchService
- Update API /context route to load active profile, support searchMode/alpha params
- Extend MCP query-docs tool with searchMode and alpha parameters
- Update settings API to work with embedding_profiles table
- Add comprehensive test coverage for profiles, registry, version scoping

Status: 445/451 tests passing, core feature complete

src/lib/server/embeddings/registry.ts

@@ -0,0 +1,64 @@
+/**
+ * Provider Registry — map providerKind to EmbeddingProvider instances.
+ *
+ * Replaces the enum-style factory with a registry pattern that supports
+ * arbitrary custom provider adapters without changing core types.
+ */
+
+import type { EmbeddingProvider } from './provider.js';
+import { NoopEmbeddingProvider } from './provider.js';
+import { OpenAIEmbeddingProvider } from './openai.provider.js';
+import { LocalEmbeddingProvider } from './local.provider.js';
+import type { EmbeddingProfile } from '../db/schema.js';
+
+export type ProviderFactory = (config: Record<string, unknown>) => EmbeddingProvider;
+
+const PROVIDER_REGISTRY: Record<string, ProviderFactory> = {
+	'local-transformers': (_config) => new LocalEmbeddingProvider(),
+	'openai-compatible': (config) =>
+		new OpenAIEmbeddingProvider({
+			baseUrl: config.baseUrl as string,
+			apiKey: config.apiKey as string,
+			model: config.model as string,
+			dimensions: config.dimensions as number | undefined,
+			maxBatchSize: config.maxBatchSize as number | undefined
+		})
+};
+
+/**
+ * Create an EmbeddingProvider from a persisted EmbeddingProfile.
+ *
+ * Falls back to NoopEmbeddingProvider when the providerKind is not recognized.
+ */
+export function createProviderFromProfile(profile: EmbeddingProfile): EmbeddingProvider {
+	const factory = PROVIDER_REGISTRY[profile.providerKind];
+	if (!factory) return new NoopEmbeddingProvider();
+	const config = (profile.config as Record<string, unknown>) ?? {};
+	return factory(config);
+}
+
+/**
+ * Return metadata for the default local profile.
+ *
+ * Used by migration seeds and runtime defaults.
+ */
+export function getDefaultLocalProfile(): Pick<
+	EmbeddingProfile,
+	'id' | 'providerKind' | 'model' | 'dimensions'
+> {
+	return {
+		id: 'local-default',
+		providerKind: 'local-transformers',
+		model: 'Xenova/all-MiniLM-L6-v2',
+		dimensions: 384
+	};
+}
+
+/**
+ * Return all registered providerKind values.
+ *
+ * Useful for settings UI validation and provider discovery.
+ */
+export function getRegisteredProviderKinds(): string[] {
+	return Object.keys(PROVIDER_REGISTRY);
+}