feat: add conversation persistence and context management layer

- SQLAlchemy async database layer (SQLite dev, PostgreSQL prod)
- Conversation and Message models with UUID primary keys
- Token counting utilities using litellm
- Context summarization at 80% token threshold
- REST API endpoints for multi-turn conversations
- 19 conversation tests, 6 token tests (176 total passing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

webber-api/src/db/__init__.py

@@ -0,0 +1,14 @@
+"""
+Database package for Webber.
+
+Provides async SQLAlchemy database access following core-api patterns.
+"""
+from src.db.database import Database, get_database, get_session
+from src.db.models import Base
+
+__all__ = [
+    "Database",
+    "get_database",
+    "get_session",
+    "Base",
+]