saved sessions: replace transient history with persistent DB storage and auto-save

- Add saved_sessions table and CRUD endpoints (save, list, resume, update, delete)
- Save is instant (no LLM summarization), stores exact conversation with tool calls
- Resume loads full message history into chat UI, sends transcript to agent on first message
- Auto-save updates DB after every agent response once a session is saved
- Delete old filesystem-based session/group management (sessions router, useChatSessions, useChatGroups)
- Clean up ChatHeader, SessionList, ChatDetailPanel for saved sessions flow

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/servers/api/upload/upload.ts

@@ -13,7 +13,6 @@ uploadRouter.post('/', async (ctx) => {
 
   const file = body.file as File | null;
   const sessionId = (body.sessionId as string) || null;
-  const provider = (body.provider as 'claude' | 'pi-mono') || null;
 
   if (!file || !(file instanceof File)) {
     return ctx.json({ error: 'file is required' }, 400);
@@ -33,8 +32,8 @@ uploadRouter.post('/', async (ctx) => {
     const filename = `${crypto.randomUUID()}.${ext}`;
 
     let saveDir: string;
-    if (sessionId && provider) {
-      saveDir = getAttachmentsDir(user.email, provider, sessionId);
+    if (sessionId) {
+      saveDir = getAttachmentsDir(user.email, sessionId);
     } else {
       saveDir = getTmpAttachmentsDir(user.email);
     }