Make OpenAI-family strict mode routable and stop redacting payee words

Strict structured-output mode rejects uniqueItems, so every request to a
gpt-5.6-family zero-data-retention endpoint failed with HTTP 400 behind
a generic error; duplicates were already rejected server-side, so the
keyword leaves the wire schemas, pinned by a strict-keyword allowlist
test built from the ledger that hit this.

The bare-BIC redaction pattern deleted every 8- and 11-letter word —
Openbank, BAUMARKT, RACETRACKER — blinding the model to the payee it
was asked to classify and tripping the unsafe-merchant check on honest
answers. BICs now die only labeled or attached to their IBAN, account
labels join the redaction secrets, an identifier-shaped merchant name
degrades to a merchant-less proposal instead of failing the row, and a
provider error inside an HTTP 200 envelope is reported as such (numeric
code only) instead of as envelope corruption.

internal/classification/propose.go

@@ -65,7 +65,7 @@ func taxonomySchema() map[string]any {
 	merchant := map[string]any{
 		"type": "object", "additionalProperties": false,
 		"required":   []string{"name", "aliases"},
-		"properties": map[string]any{"name": name, "aliases": map[string]any{"type": "array", "maxItems": 32, "uniqueItems": true, "items": name}},
+		"properties": map[string]any{"name": name, "aliases": map[string]any{"type": "array", "maxItems": 32, "items": name}},
 	}
 	return map[string]any{
 		"type": "object", "additionalProperties": false,