✂️ Implement message chunking mechanism for NIP-44 size limit compliance

This ensures all messages can be properly encrypted and transmitted regardless of size.

Fixes issue #2

migrations/4-createChunksStore.sql

@@ -0,0 +1,13 @@
+CREATE TABLE event_chunks (
+    chunk_id INTEGER PRIMARY KEY AUTOINCREMENT,
+    message_id TEXT NOT NULL,
+    chunk_index INTEGER NOT NULL,
+    total_chunks INTEGER NOT NULL,
+    chunk_data TEXT NOT NULL,
+    conversation_key TEXT NOT NULL,
+    created_at INTEGER NOT NULL,
+    UNIQUE(message_id, chunk_index)
+);
+
+CREATE INDEX idx_event_chunks_message_id ON event_chunks(message_id);
+CREATE INDEX idx_event_chunks_created_at ON event_chunks(created_at);