Format preview worker logs like the main process, tagged with worker pid

Worker stderr is now inherited by the parent, so its log lines land in the
server log — but they arrived with the default logging format and a noisy
"preview-worker config=..." line at every spawn.

- Extract the emoji level-prefix formatting from cista.sanic_logging into
  cista.util.logformat, which has no Sanic dependency (the worker must not
  import Sanic: import-time prints could corrupt the stdout protocol).
- Worker configures its stderr handler with the same emoji prefixes plus a
  worker[pid] tag, and the config-loaded info message is removed.

cista/util/logformat.py

@@ -0,0 +1,37 @@
+"""Shared log formatting helpers with no Sanic dependency.
+
+Used by the main process (cista.sanic_logging) and by the preview worker
+subprocess, which must not import Sanic.
+"""
+
+import logging
+import unicodedata
+
+LEVEL_EMOJI = {
+    logging.DEBUG: "🔍",
+    logging.INFO: "ℹ️",  # noqa: RUF001
+    logging.WARNING: "⚠️",
+    logging.ERROR: "🛑",
+    logging.CRITICAL: "🛑",
+}
+
+
+def display_width(text: str) -> int:
+    return sum(
+        1 + (unicodedata.east_asian_width(c) in "FW")
+        for c in text
+        if unicodedata.category(c) != "Mn"
+    )
+
+
+def format_level_prefix(levelno: int) -> str:
+    emoji = LEVEL_EMOJI.get(levelno, "▪️")
+    prefix = f"{emoji} "
+    return prefix + (" " * max(0, 3 - display_width(prefix)))
+
+
+class EmojiFormatter(logging.Formatter):
+    """Compact formatter: emoji + message, no timestamp/level text/logger name."""
+
+    def format(self, record: logging.LogRecord) -> str:
+        return format_level_prefix(record.levelno) + record.getMessage()