feat: add flat-file database backend for message history

Add pluggable storage backend abstraction (vtable) to the database layer,
allowing selection between SQLite (default) and a new flat-file backend
that stores messages as human-readable plain text files.

New files:
- database_sqlite.c: extracted SQLite backend from database.c
- database_flatfile.c: plain text backend with tolerant parser,
  LMC correction chains, UTF-8/BOM/CRLF handling, integrity checks

Commands:
- /privacy logging flatfile — switch to flat-file backend
- /history verify [<jid>] — check integrity of stored history

Fixes:
- Add missing 'off' guard in flatfile backend
- Enable CHLOG/HISTORY prefs when switching to flatfile mode

Logs stored in ~/.local/share/profanity/flatlog/{account}/{contact}/{date}.log
Format: {ISO8601} [{type}|{enc}|id:{id}|corrects:{id}] {sender}: {message}

Updated: CHANGELOG, CONTRIBUTING.md, profrc.example, man page, cmd_defs,
Makefile.am, test stubs, functional test support (PROF_FLATFILE=1)

Makefile.am

@@ -3,6 +3,8 @@ core_sources = \
 	src/log.c src/common.c \
 	src/chatlog.c src/chatlog.h \
 	src/database.h src/database.c \
+	src/database_sqlite.c \
+	src/database_flatfile.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 \
@@ -329,6 +331,26 @@ check-functional-parallel: tests/functionaltests/functionaltests
 	grep -E 'PASSED|FAILED|Running' $(builddir)/test-logs/group*.log || true; \
 	if [ $$failed -ne 0 ]; then echo "FUNCTIONAL TESTS FAILED"; exit 1; fi; \
 	echo "All functional test groups passed!"
+
+# Run functional tests with the flat-file database backend
+# Usage: make check-functional-flatfile
+check-functional-flatfile: tests/functionaltests/functionaltests
+	@echo "Running functional tests with flat-file backend ($(words $(FUNC_TEST_GROUPS)) groups)..."
+	@mkdir -p $(builddir)/test-logs $(builddir)/test-files
+	@pids=""; \
+	for g in $(FUNC_TEST_GROUPS); do \
+		PROF_FLATFILE=1 ./tests/functionaltests/functionaltests $$g > $(builddir)/test-logs/group$$g-flatfile.log 2>&1 & \
+		pids="$$pids $$!"; \
+	done; \
+	failed=0; i=1; \
+	for pid in $$pids; do \
+		wait $$pid || { echo "Group $$i FAILED (flatfile)"; cat $(builddir)/test-logs/group$$i-flatfile.log; failed=1; }; \
+		i=$$((i + 1)); \
+	done; \
+	echo "=== Flat-file Test Results Summary ==="; \
+	grep -E 'PASSED|FAILED|Running' $(builddir)/test-logs/group*-flatfile.log || true; \
+	if [ $$failed -ne 0 ]; then echo "FLAT-FILE FUNCTIONAL TESTS FAILED"; exit 1; fi; \
+	echo "All flat-file functional test groups passed!"
 endif
 endif