database_flatfile: single-file storage with sparse index

Replace per-contact directory structure with a single flat file per contact.
Each file uses pipe-delimited records with a sparse byte-offset index
that is rebuilt on demand and cached in memory.

- Rewrite _ff_write_record / _ff_read_records for single-file format
- Add sparse index (every Nth record) for O(log N) seek on history read
- Cursor-based pagination: _ff_get_previous_chat uses saved byte offset
- LMC corrections applied in-place during read (_ff_apply_lmc)
- Newline escaping (\n) in message bodies for line-based storage
- Simplify verify: parse + timestamp order + LMC reference check
- Update parser helpers for new field layout

src/database_flatfile.h

@@ -28,6 +28,7 @@
 
 #include <glib.h>
 #include <stdio.h>
+#include <sys/types.h>
 
 #include "database.h"
 #include "xmpp/xmpp.h"
@@ -60,8 +61,44 @@ typedef struct
     char* from_jid;
     char* from_resource;
     char* message;
+    off_t file_offset;
 } ff_parsed_line_t;
 
+// --- Sparse index for single-file lookup ---
+
+#define FF_INDEX_STEP 500
+
+typedef struct
+{
+    off_t byte_offset;
+    gint64 timestamp_epoch;
+} ff_index_entry_t;
+
+typedef struct
+{
+    time_t mtime;
+    off_t size;
+    ino_t inode;
+} ff_file_stamp_t;
+
+typedef struct
+{
+    char* filepath;
+    ff_index_entry_t* entries;
+    size_t n_entries;
+    size_t cap_entries;
+    size_t total_lines;
+    ff_file_stamp_t stamp;
+    off_t cursor_offset;
+    int bom_len;
+} ff_contact_state_t;
+
+// State management
+ff_contact_state_t* ff_state_new(const char* filepath);
+void ff_state_free(ff_contact_state_t* state);
+gboolean ff_state_ensure_fresh(ff_contact_state_t* state);
+off_t ff_state_offset_for_time(ff_contact_state_t* state, const char* iso_time);
+
 // --- Type conversion helpers ---
 
 const char* ff_get_message_type_str(prof_msg_type_t type);
@@ -73,7 +110,7 @@ prof_enc_t ff_get_message_enc_type(const char* const encstr);
 
 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);
+char* ff_get_log_path(const char* contact_barejid);
 gboolean ff_ensure_dir(const char* path);
 
 // --- Escape / unescape ---