scan: create the database's directory where its path leads (closes #119)

scan created a missing database directory from filepath.Dir of the
path, which cleans it as text and drops each ".." together with the
name before it. After a symlinked directory that is another directory
than the one the system reaches when it opens the file, so scan left an
empty directory beside the symlink and then failed to create the
database. lockScanDatabase and openScanDatabase now both call
createDatabaseDirectory, beside databasePath, which cuts the last
element off the path as text and creates what is missing along it. A
comment in databasePath now says why the working directory has its
symlinks resolved.

Unverified: a missing directory before a ".." in the path is created along the way; no test covers it.

Model: opus-5-5

TODO.md

@@ -29,6 +29,14 @@
 
 # Completed Steps
 
+- `scan` creates the database's directory where the database path leads: the
+  path with its last element cut off as text, never cleaned, so a `..` after a
+  symlinked directory is followed through the symlink, as when the file is
+  opened. With `SFDUPES_DATABASE=/srv/link/../new/db.sqlite` and `/srv/link` a
+  symlink to `/data/x`, it creates `/data/new` and the database in it, where it
+  used to create `/srv/new` and then fail (2026-10-17,
+  https://git.eeqj.de/sneak/sfdupes/issues/119)
+
 - `scan` never records its own database: once the database and its `.lock` file
   are open, it notes the device and inode of the database file, its `-wal` and
   `-shm` files and its `.lock` file, and the walk passes over a file with one of