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)

src/database_flatfile.h

@@ -0,0 +1,110 @@
+/*
+ * database_flatfile.h
+ * vim: expandtab:ts=4:sts=4:sw=4
+ *
+ * Copyright (C) 2026 Profanity Contributors
+ *
+ * This file is part of Profanity.
+ *
+ * Profanity is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Profanity is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Profanity.  If not, see <https://www.gnu.org/licenses/>.
+ *
+ * Internal header shared between database_flatfile*.c modules.
+ * Not part of the public API — do not include from outside the flatfile backend.
+ */
+
+#ifndef DATABASE_FLATFILE_H
+#define DATABASE_FLATFILE_H
+
+#include <glib.h>
+#include <stdio.h>
+
+#include "database.h"
+#include "xmpp/xmpp.h"
+#include "xmpp/message.h"
+
+// --- Constants ---
+
+#define DIR_FLATLOG      "flatlog"
+#define FLATFILE_HEADER  "# profanity chat log — UTF-8, LF line endings\n# vim: set fileencoding=utf-8 fileformat=unix :\n"
+#define FF_MAX_LINE_LEN  (10 * 1024 * 1024) /* 10 MB — reject lines longer than this */
+#define FF_MAX_LMC_DEPTH 100                /* max correction chain depth */
+
+// --- Shared global ---
+
+// Account JID stored during init for path construction.
+// Defined in database_flatfile.c, used by all flatfile modules.
+extern char* g_flatfile_account_jid;
+
+// --- Parsed line structure ---
+
+typedef struct
+{
+    char* timestamp_str;
+    GDateTime* timestamp;
+    char* type;
+    char* enc;
+    char* stanza_id;
+    char* archive_id;
+    char* replace_id;
+    char* from_jid;
+    char* from_resource;
+    char* message;
+} ff_parsed_line_t;
+
+// --- Type conversion helpers ---
+
+const char* ff_get_message_type_str(prof_msg_type_t type);
+prof_msg_type_t ff_get_message_type_type(const char* const type);
+const char* ff_get_message_enc_str(prof_enc_t enc);
+prof_enc_t ff_get_message_enc_type(const char* const encstr);
+
+// --- Path helpers ---
+
+char* ff_jid_to_dir(const char* jid);
+char* ff_get_contact_dir(const char* contact_barejid);
+char* ff_get_log_path(const char* contact_barejid, GDateTime* dt);
+gboolean ff_ensure_dir(const char* path);
+
+// --- Escape / unescape ---
+
+char* ff_escape_message(const char* text);
+char* ff_unescape_message(const char* text);
+char* ff_escape_meta_value(const char* val);
+char* ff_unescape_meta_value(const char* val);
+
+// --- I/O ---
+
+char* ff_readline(FILE* fp, gboolean* truncated);
+void ff_write_line(FILE* fp, const char* timestamp, const char* type, const char* enc,
+                   const char* stanza_id, const char* archive_id, const char* replace_id,
+                   const char* from_jid, const char* from_resource, const char* message_text);
+
+// --- Parser helpers ---
+
+const char* ff_find_unescaped_char(const char* str, char ch);
+char** ff_split_meta(const char* meta);
+const char* ff_find_unescaped_colonspace(const char* str);
+char* ff_unescape_sender_resource(const char* res);
+
+// --- Parser ---
+
+void ff_parsed_line_free(ff_parsed_line_t* pl);
+ff_parsed_line_t* ff_parse_line(const char* line);
+ProfMessage* ff_parsed_to_profmessage(ff_parsed_line_t* pl);
+
+// --- Integrity verification (database_flatfile_verify.c) ---
+
+GSList* ff_verify_integrity(const gchar* const contact_barejid);
+
+#endif