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/db/schema.ts

@@ -1,4 +1,4 @@
-import { blob, integer, real, sqliteTable, text } from 'drizzle-orm/sqlite-core';
+import { blob, integer, primaryKey, real, sqliteTable, text } from 'drizzle-orm/sqlite-core';
 
 // ---------------------------------------------------------------------------
 // repositories
@@ -86,18 +86,41 @@ export const snippets = sqliteTable('snippets', {
 	createdAt: integer('created_at', { mode: 'timestamp' }).notNull()
 });
 
+// ---------------------------------------------------------------------------
+// embedding_profiles
+// ---------------------------------------------------------------------------
+export const embeddingProfiles = sqliteTable('embedding_profiles', {
+	id: text('id').primaryKey(),
+	providerKind: text('provider_kind').notNull(),
+	title: text('title').notNull(),
+	enabled: integer('enabled', { mode: 'boolean' }).notNull().default(true),
+	isDefault: integer('is_default', { mode: 'boolean' }).notNull().default(false),
+	model: text('model').notNull(),
+	dimensions: integer('dimensions').notNull(),
+	config: text('config', { mode: 'json' }).notNull().$type<Record<string, unknown>>(),
+	createdAt: integer('created_at').notNull(),
+	updatedAt: integer('updated_at').notNull()
+});
+
 // ---------------------------------------------------------------------------
 // snippet_embeddings
 // ---------------------------------------------------------------------------
-export const snippetEmbeddings = sqliteTable('snippet_embeddings', {
-	snippetId: text('snippet_id')
-		.primaryKey()
-		.references(() => snippets.id, { onDelete: 'cascade' }),
-	model: text('model').notNull(), // embedding model identifier
-	dimensions: integer('dimensions').notNull(),
-	embedding: blob('embedding').notNull(), // Float32Array as binary blob
-	createdAt: integer('created_at', { mode: 'timestamp' }).notNull()
-});
+export const snippetEmbeddings = sqliteTable(
+	'snippet_embeddings',
+	{
+		snippetId: text('snippet_id')
+			.notNull()
+			.references(() => snippets.id, { onDelete: 'cascade' }),
+		profileId: text('profile_id')
+			.notNull()
+			.references(() => embeddingProfiles.id, { onDelete: 'cascade' }),
+		model: text('model').notNull(), // embedding model identifier
+		dimensions: integer('dimensions').notNull(),
+		embedding: blob('embedding').notNull(), // Float32Array as binary blob
+		createdAt: integer('created_at').notNull()
+	},
+	(table) => [primaryKey({ columns: [table.snippetId, table.profileId] })]
+);
 
 // ---------------------------------------------------------------------------
 // indexing_jobs
@@ -165,6 +188,9 @@ export type NewDocument = typeof documents.$inferInsert;
 export type Snippet = typeof snippets.$inferSelect;
 export type NewSnippet = typeof snippets.$inferInsert;
 
+export type EmbeddingProfile = typeof embeddingProfiles.$inferSelect;
+export type NewEmbeddingProfile = typeof embeddingProfiles.$inferInsert;
+
 export type SnippetEmbedding = typeof snippetEmbeddings.$inferSelect;
 export type NewSnippetEmbedding = typeof snippetEmbeddings.$inferInsert;