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/preview_worker.py

@@ -17,6 +17,7 @@ import gc
 import io
 import logging
 import mimetypes
+import os
 import shlex
 import struct
 import subprocess
@@ -33,9 +34,19 @@ import pyvips
 from blake3 import blake3
 
 from cista import config
+from cista.util.logformat import format_level_prefix
 
 logger = logging.getLogger(__name__)
 
+
+class _WorkerLogFormatter(logging.Formatter):
+    """Emoji level prefix like the main process, tagged with the worker pid."""
+
+    def format(self, record: logging.LogRecord) -> str:
+        prefix = format_level_prefix(record.levelno)
+        return f"{prefix}worker[{os.getpid()}]: {record.getMessage()}"
+
+
 AVIF_FAST_EFFORT = 0
 
 DOC_PREVIEW_SUFFIXES = {".pdf", ".xps", ".epub", ".mobi"}
@@ -554,11 +565,14 @@ def _run_loop() -> None:
 
 
 def main() -> None:
-    # Configure all log output to stderr before any imports that may emit logs.
-    logging.basicConfig(stream=sys.stderr, level=logging.INFO)
+    # Configure all log output to stderr before any imports that may emit
+    # logs. stderr is inherited by the parent, so this lands in the server
+    # log, formatted like the main process and tagged with the worker pid.
+    handler = logging.StreamHandler(sys.stderr)
+    handler.setFormatter(_WorkerLogFormatter())
+    logging.basicConfig(level=logging.INFO, handlers=[handler])
     try:
         config.load_config()
-        logger.info("preview-worker config=%s", config.conffile)
     except Exception:
         logger.exception("preview-worker failed to load config at startup")
     if len(sys.argv) > 1:

cista/sanic_logging.py

@@ -3,11 +3,13 @@
 import logging
 import os
 import sys
-import unicodedata
 from ipaddress import IPv6Address
 
 from sanic.log import LOGGING_CONFIG_DEFAULTS
 
+from cista.util.logformat import EmojiFormatter as _EmojiFormatter
+from cista.util.logformat import display_width as _display_width
+
 logger = logging.getLogger("cista.access")
 
 
@@ -132,14 +134,6 @@ def format_duration_ms(duration_ms: float) -> str:
     return f"{hours}h{minutes}m"
 
 
-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_left(label: str) -> str:
     return label[:19].ljust(19)
 
@@ -279,28 +273,6 @@ def configure_access_logging() -> None:
     logger.propagate = False
 
 
-_LEVEL_EMOJI = {
-    logging.DEBUG: "🔍",
-    logging.INFO: "ℹ️",  # noqa: RUF001
-    logging.WARNING: "⚠️",
-    logging.ERROR: "🛑",
-    logging.CRITICAL: "🛑",
-}
-
-
-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()
-
-
 def configure_main_logging() -> None:
     """Replace Sanic's verbose 'Main yyyy-mm-dd INFO:' prefix with emoji-only format.
 

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()