docs: document logging config

doc/hosting/environment-variables.md

@@ -235,3 +235,45 @@ This setting is useful for developers working on PixivFE itself or for troublesh
 **Default**: `/tmp/pixivfe/responses`
 
 Defines where responses from the Pixiv API are saved when in development mode.
+
+## Logging Configuration
+
+PixivFE uses [uber-go/zap](https://github.com/uber-go/zap) for structured, leveled logging. The following environment variables control the logging behavior.
+
+### `PIXIVFE_LOG_LEVEL`
+
+**Required**: No
+
+**Default:** `info`
+
+Sets the minimum level of log messages to output. Valid options are:
+
+- `debug`: Includes all log messages, including detailed debug information.
+- `info`: Includes informational messages, warnings, and errors.
+- `warn`: Includes only warning and error messages.
+- `error`: Includes only error messages.
+
+### `PIXIVFE_LOG_OUTPUTS`
+
+**Required**: No
+
+**Default:** `["stdout"]`
+
+Specifies where log messages should be sent. This can be a single output or multiple outputs separated by commas. Valid options include:
+
+- `stdout`: Standard output (console)
+- `stderr`: Standard error
+- File paths: e.g., `/var/log/pixivfe.log`
+
+Example: `PIXIVFE_LOG_OUTPUTS=stdout,/var/log/pixivfe.log`
+
+### `PIXIVFE_LOG_FORMAT`
+
+**Required**: No
+
+**Default:** `console`
+
+Determines the format of log messages. Valid options are:
+
+- `console`: Human-readable format suitable for console output
+- `json`: Structured JSON format, useful for log parsing and analysis tools