feat: enhance memory system with tiktoken and improved prompt guidelines

Add accurate token counting using tiktoken library and significantly enhance
memory update prompts with detailed section guidelines, multilingual support,
and improved fact extraction. Update deep-research skill to be more proactive
for research queries.

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

backend/uv.lock

@@ -1,5 +1,5 @@
 version = 1
-revision = 3
+revision = 2
 requires-python = ">=3.12"
 resolution-markers = [
     "python_full_version >= '3.14' and sys_platform == 'win32'",
@@ -620,6 +620,7 @@ dependencies = [
     { name = "readabilipy" },
     { name = "sse-starlette" },
     { name = "tavily-python" },
+    { name = "tiktoken" },
     { name = "uvicorn", extra = ["standard"] },
 ]
 
@@ -651,6 +652,7 @@ requires-dist = [
     { name = "readabilipy", specifier = ">=0.3.0" },
     { name = "sse-starlette", specifier = ">=2.1.0" },
     { name = "tavily-python", specifier = ">=0.7.17" },
+    { name = "tiktoken", specifier = ">=0.8.0" },
     { name = "uvicorn", extras = ["standard"], specifier = ">=0.34.0" },
 ]