Name the scan lock file from the database's resolved path (closes #101)

The lock file was the configured database path with .lock appended, so
a scan reaching the database through a symlink to it locked a different
file and ran alongside a scan holding the lock through the real path.
Before it takes the lock, scan now creates the database file, empty, if
it is missing, following a symlink to its target as SQLite would, and
names the lock file from that file's path with symlinks resolved. A
scan started before the database exists therefore takes the same lock
file as every later scan through the same path. testDBPath resolves its
temp dir, so lock errors name the expected path where the temp dir is
itself reached through a symlink.

Model: opus-5-5

TODO.md

@@ -29,6 +29,11 @@
 
 # Completed Steps
 
+- the scan lock file is named from the database's path with symlinks resolved,
+  after `scan` creates the database file if it is missing, so a scan through a
+  symlink to the database fails while another scan holds the lock (2026-10-14,
+  https://git.eeqj.de/sneak/sfdupes/issues/101)
+
 - on a terminal, a warning for the last file of a phase no longer erases the
   phase's finished bar: the bar is drawn again below the warning (2026-10-14,
   https://git.eeqj.de/sneak/sfdupes/issues/102)