feat(logger): report errors to Sentry when SENTRY_DSN is set

[juliusknorr]

Part of Euro-Office/DocumentServer#392 (stage 2).

• Common/sources/reporters/sentry.js: log4js appender for level error+, @sentry/node 10
• logger.js adds the appender when SENTRY_DSN is set (in code rather than via entrypoints, so pkg bundles it); disabled otherwise
• Common/sources/reporters/index.js is the only entry point: init(service) initializes all backends, appenders() returns their log4js appenders. Adding a backend = one file plus one line there
• Initialized first in DocService, FileConverter, SpellChecker, AdminPanel; events carry a service tag (docservice, converter, spellchecker, adminpanel)
• Config via SENTRY_DSN, SENTRY_ENVIRONMENT, SENTRY_RELEASE (default documentserver@<version>)
• Groups by format template, passes Error objects through
• Tags tenant and docId from the log context (placeholders skipped); userId and Nextcloud ids follow in stage 3
• Flushes on logger.shutdown. Unhandled rejections already reach the existing uncaughtException handlers (Node's default throw mode), so the SDK's own crash handlers are disabled. AdminPanel gets the same handler.
• Scrubs token=/doc= query params, Authorization headers and bearer tokens; sendDefaultPii: false

Tested: unit test in tests/unit/sentryReporter.tests.js, plus a smoke run of the real SDK against a local sink (scrubbing, grouping and flush before exit).

AI-assisted, see Assisted-by trailer.