feat(pii): extend a detection over the previous word (extend_to_previous_word)

The mirror of extend_to_next_word. A name model often tags the surname with
high confidence and scores the first name in front of it below min_score,
so "Gast Greta Waldmeister" became "Gast Greta [REDACTED:ner:LASTNAME]".

extend_to_previous_word lists entity groups whose accepted detections also
cover the word directly before them on the same line. The preceding word
must consist of letters (plus '-' and the apostrophe): a word with digits -
an amount, a date, a room number - is never taken, nor is a protected term,
and the search never crosses a line break.

Docs: operations/middleware.md and advanced/model-configuration.md.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Stefan Walcz <stefan.walcz@walcz.de>

docs/content/advanced/model-configuration.md

@@ -1111,6 +1111,8 @@ PII redaction is NER-based and runs on the **request** (input) side. It has two
       EMAIL: mask
     extend_to_next_word:      # optional: also mask the word right after these
       - ZIPCODE               # groups on the same line (e.g. the town after a postal code)
+    extend_to_previous_word:  # optional: also mask the letters-only word right before these
+      - LASTNAME              # groups on the same line (e.g. the first name before a surname)
     protected_terms:          # optional: values that must pass unchanged even if tagged
       - Hotel Seeblick        # (plus protected_terms_files: one term per line, in the models path)
   ```

docs/content/operations/middleware.md

@@ -121,6 +121,20 @@ A word is a run of letters, digits, combining marks, `-`, `'` and inner dots
 line break, and it does nothing when no word follows. Only one word is added,
 so a multi-word town ("Frankfurt am Main") keeps its tail.
 
+`extend_to_previous_word` is the mirror for the word *before* a detection. A
+name model often tags the surname with high confidence and scores the first
+name in front of it below `min_score` (`Gast Greta [REDACTED:ner:LASTNAME]`):
+
+```yaml
+pii_detection:
+  extend_to_previous_word:
+    - LASTNAME                # "Gast Greta Waldmeister" -> "Gast [REDACTED:ner:LASTNAME]"
+```
+
+Here the preceding word must consist of letters (plus `-` and `'`): a word
+with digits - an amount, a date, a room number - is never taken, nor a
+protected term. The search skips spaces and tabs but never a line break.
+
 #### Protected terms
 
 The opposite failure is a value the model tags although it must reach the