fix(analyze): fold document output to ISO 8859-15 for Medical Office paste

German practice software (INDAMED MEDICAL OFFICE) stores free text in
ISO 8859-15, an 8-bit code. Pasting our cleaned document there rendered
typographic Unicode the LLM emits — en-/em-dashes, curly quotes,
ellipses, fraction slashes — as replacement boxes, while umlauts, the
micro sign, guillemets and the euro sign (all within 8859-15) pasted
fine. The KBV xDT data standard confirms ISO 8859-15 as the canonical
charset for German practice systems.

Add `analyze::charset::fold_to_iso8859_15`, applied in the worker right
after the gazetteer and before `content_md` is persisted: representable
chars pass through verbatim, the residual is transliterated via the
`deunicode` table (en-dash -> '-', ellipsis -> '...', '1/2' -> "1/2"),
and unmapped chars are left untouched rather than dropped. Because
representable chars never reach deunicode, its ASCII-only nature does
not flatten umlauts or the micro sign.

refs #13

server/Cargo.lock

@@ -319,6 +319,12 @@ dependencies = [
  "powerfmt",
 ]
 
+[[package]]
+name = "deunicode"
+version = "1.6.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "abd57806937c9cc163efc8ea3910e00a62e2aeb0b8119f1793a978088f8f6b04"
+
 [[package]]
 name = "displaydoc"
 version = "0.2.5"
@@ -347,6 +353,7 @@ dependencies = [
  "axum",
  "axum-extra",
  "bcrypt",
+ "deunicode",
  "doctate-common",
  "dotenvy",
  "filetime",