Name the database each command uses on stderr (closes #111)

scan, report and trees now print one line on stderr before anything
else, naming the database and whether its path is the default or comes
from SFDUPES_DATABASE; scan also says when the file is new. report and
trees print it before opening the database, so a missing-database error
follows it. A relative path gets the working directory put in front of
it, with its symlinks resolved as the kernel reports it, and is not
cleaned: a ".." after a symlink names another file than the cleaned
path does. README "Database" lists when scan writes each file.

Unverified: the line on a terminal has no test of its own.

Model: opus-5-5

TODO.md

@@ -29,6 +29,12 @@
 
 # Completed Steps
 
+- `scan`, `report` and `trees` first print a line on stderr naming the database
+  they use, as an absolute path, and whether that path is the default or comes
+  from `SFDUPES_DATABASE`; `scan` says when the database is new. README
+  "Database" says when `scan` writes each file (2026-10-16,
+  https://git.eeqj.de/sneak/sfdupes/issues/111)
+
 - re-vendor the canonical files and model scripts from `sneak/prompts` `next` at
   `e3f79c4`: golangci-lint v2.14.0; lint and test are phases of the `Dockerfile`
   that write no image, and `make test` runs the suite under the race detector,