feat: multi-user isolation and graceful shutdown

- Add chat_id to events, settings, and days_off for per-user data
- Add proper graceful shutdown (signal handling, WaitGroup)
- Separate polling and webhook modes with goroutine management
- Add schema migration from single-user to multi-user schema
- Refactor handlers with shared actionFunc pattern (msg + callback)
- Fix schema path for Docker deployment (/app/db/schema.sql)
- Remove emoji from output text for cleaner formatting

db/schema.sql

@@ -1,26 +1,26 @@
 CREATE TABLE IF NOT EXISTS events (
-    id INTEGER PRIMARY KEY,
-    event_type TEXT NOT NULL
-        CHECK (event_type IN ('in', 'out')),
-    occurred_at INTEGER NOT NULL UNIQUE
-          CHECK (occurred_at > 0),
-      note TEXT NOT NULL DEFAULT '',
-    created_at INTEGER NOT NULL
-          DEFAULT (unixepoch())
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    chat_id INTEGER NOT NULL,
+    event_type TEXT NOT NULL CHECK (event_type IN ('in', 'out')),
+    occurred_at INTEGER NOT NULL CHECK (occurred_at > 0),
+    note TEXT NOT NULL DEFAULT '',
+    created_at INTEGER NOT NULL DEFAULT (unixepoch())
 );
 
-CREATE INDEX IF NOT EXISTS idx_events_occurred_at
-ON events(occurred_at);
+CREATE INDEX IF NOT EXISTS idx_events_chat_occurred
+ON events(chat_id, occurred_at);
 
 CREATE TABLE IF NOT EXISTS settings (
-    key   TEXT PRIMARY KEY,
-    value TEXT NOT NULL DEFAULT ''
+    chat_id INTEGER NOT NULL,
+    key TEXT NOT NULL,
+    value TEXT NOT NULL DEFAULT '',
+    PRIMARY KEY (chat_id, key)
 );
 
 CREATE TABLE IF NOT EXISTS days_off (
-    date TEXT PRIMARY KEY
-          CHECK (date = strftime('%Y-%m-%d', date)),
+    chat_id INTEGER NOT NULL,
+    date TEXT NOT NULL CHECK (date = strftime('%Y-%m-%d', date)),
     reason TEXT NOT NULL DEFAULT '',
-    created_at INTEGER NOT NULL
-          DEFAULT (unixepoch())
+    created_at INTEGER NOT NULL DEFAULT (unixepoch()),
+    PRIMARY KEY (chat_id, date)
 );