Add web search enrichment feature with source persistence and UI integration

Introduce optional web search enrichment flow for chat and regenerate requests. New /websearch endpoint calls enrich_prompt via SearXNG and returns enriched_prompt + citation sources.

DB: add sources_json column to chat_messages via ensure_sources_column migration helper.

Backend: persist sources_json for assistant replies (streaming and non-streaming); extend ChatRequest/RegenerateRequest to accept enriched_message and sources; history endpoint returns sources.

Frontend: add toggle for web search, settings for SearXNG URL + engines, and optional enrichment calls in sendMessage/regenerate. Render citation sources as rounded chips labeled with base domain under assistant replies.

Dependencies: add beautifulsoup4, httpx[http2], numpy for enrichment pipeline.

backend/database.py

@@ -1,6 +1,20 @@
 
 from sqlalchemy import create_engine
 from sqlalchemy.orm import sessionmaker, DeclarativeBase
+from sqlalchemy import text
+
+"""
+Database utilities and configuration.  This module defines the SQLAlchemy
+engine, session factory and base class for models.  It also contains a
+lightweight migration helper used to evolve the schema over time.  The
+`ensure_sources_column` helper adds a new `sources_json` column to the
+`chat_messages` table if it does not already exist.  This is required
+for persisting citation sources alongside assistant messages.
+
+The migration uses SQLite's `ALTER TABLE` syntax and therefore should
+only run once on startup.  It is safe to call repeatedly: when the
+column already exists, the function will simply no‑op.
+"""
 
 DATABASE_URL = "sqlite:///./backend/app.db"
 
@@ -12,3 +26,13 @@ SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
 
 class Base(DeclarativeBase):
     pass
+
+
+def ensure_sources_column(engine):
+    try:
+        with engine.connect() as conn:
+            cols = [row[1] for row in conn.execute(text("PRAGMA table_info(chat_messages)"))]
+            if "sources_json" not in cols:
+                conn.execute(text("ALTER TABLE chat_messages ADD COLUMN sources_json TEXT DEFAULT '[]'"))
+    except Exception as e:
+        print("[db] ensure_sources_column error:", e)