refactor: split database_flatfile.c into functional modules

Split the monolithic database_flatfile.c into:
- database_flatfile.h: internal header with shared types and prototypes
- database_flatfile_parser.c: parsing, escaping, IO helpers
- database_flatfile_verify.c: integrity verification logic
- database_flatfile.c: core backend (init, write, read, LMC, vtable)

Makefile.am

@@ -4,7 +4,9 @@ core_sources = \
 	src/chatlog.c src/chatlog.h \
 	src/database.h src/database.c \
 	src/database_sqlite.c \
-	src/database_flatfile.c \
+	src/database_flatfile.c src/database_flatfile.h \
+	src/database_flatfile_parser.c \
+	src/database_flatfile_verify.c \
 	src/log.h src/profanity.c src/common.h \
 	src/profanity.h src/xmpp/chat_session.c \
 	src/xmpp/chat_session.h src/xmpp/muc.c src/xmpp/muc.h src/xmpp/jid.h src/xmpp/jid.c \