Record what nobody can find, and a spelling corrector that refuses to guess

Two halves of one question - how to handle a misspelled search - deliberately
kept separate, because only one of them should be turned on today.

WHAT NOBODY CAN FIND IS NOT RECORDED ANYWHERE. A search returning zero results
is the only evidence of the gap between what people look for and what is there,
and it vanished. Logged now at INFO with a stable prefix, so a week of it greps
into a list. That list is what should decide whether correction is worth wiring
in: multi-word matching and cross-field matching both changed in the last day,
so a good share of what used to fail may already be found. The failures left
over might be typos, or vocabulary nobody has entered, or records that genuinely
do not exist - and each wants a different answer.

THE CORRECTOR IS BUILT AND NOT WIRED IN. shopdb/core/services/spellfix.py, with
tests, ready to attach in about ten lines once there is evidence about what to
attach it to.

NOT SOUNDEX, which was the obvious candidate. MySQL has it and SQLite does not,
and the suite runs on SQLite while production runs MySQL - so a SOUNDEX() in a
query is either an error in every test or a production path no test executes.
It is also wrong for this data: soundex is English-name phonetics, four
characters wide, and it DISCARDS DIGITS, so CSF16 and CSF17 hash identically.
Half of what people search here is an identifier.

So: character distance in Python, same behaviour on both dialects.

THE VOCABULARY IS THE DATA. No dictionary holds Genspect, Telesis, Keyence or
wax-trace. Terms come from the columns being searched, which also means a vendor
added this morning is correctable this morning.

TWO GUARDS, and they are the point rather than a detail. Digits must match
EXACTLY: CSF16 to CSF17 is one edit and a different bay, so anything carrying
digits is either right or not correctable - while cfs16 to csf16 still works,
because the guard is on the digits and not on identifiers wholesale. And the
first character must match, since typos land mid-word far more often than on the
first key, which costs almost no recall.

Distance is Damerau-Levenshtein so a transposition costs one edit rather than
two - Keyecne for Keyence is the commonest error there is, and plain Levenshtein
pushes it past the threshold on short words. Allowed distance scales with
length. A tie returns NOTHING: two equally good candidates means there is no
answer, and offering either implies a confidence that is not there.

It suggests; it never rewrites. Silently searching for something else is how
somebody orders the wrong cartridge.

13 tests, weighted toward the refusals, because those are the cases where being
wrong costs something.

shopdb/core/api/search.py

@@ -1099,6 +1099,16 @@ def global_search():
 
     total_all = len(unique_results)
 
+    # A search that found NOTHING is the only evidence of what people cannot
+    # find, and it is not otherwise recorded anywhere. Logged at INFO with a
+    # stable prefix so a week of it can be grepped into a list, which is what
+    # should decide whether spelling correction is worth wiring in - the recent
+    # multi-word and cross-field matching changes may already have fixed a good
+    # share of what used to fail. Guessing at that would be building for a
+    # problem nobody has measured.
+    if not unique_results:
+        logger.info('search-no-results query=%r', query)
+
     # Limit total results
     unique_results = unique_results[:50]