Allow configuring a dedicated OCR user

[R0Wi]

Summary

• Fixes Automated OCR operations are attributed to the impersonated file owner (audit-trail / accountability concern) #385: automated OCR operations were always attributed to the impersonated file owner, making them indistinguishable from manual edits in the version history and activity log.
• Adds an optional global admin setting ("OCR user"). If left empty (default), behavior is unchanged and the file owner is impersonated as before. If set, the whole OCR run (user session, filesystem access, resulting file version/label) runs as that configured user instead.
• Success notifications are still always sent to the file owner. If the configured user doesn't exist, the app logs a warning and falls back to the file owner.

Changes

• lib/Model/GlobalSettings.php: new nullable processingUserId setting.
• lib/Service/OcrService.php: reads global settings before initializing the user environment, determines the processing uid (configured user, or file owner as fallback), and separates the "processing user" from the "owner" (used for notifications).
• lib/Service/GlobalSettingsService.php: empty string now maps to null for nullable string settings, mirroring the existing 0 -> null behavior for ints.
• lib/Controller/GlobalSettingsController.php: trims and normalizes blank input to null.
• src/components/GlobalSettings.vue: new text input for the OCR user, saved on blur.
• README.md: documents the new setting and its permission requirements.

Test plan

• Added OcrServiceTest::testRunsOcrProcessWithConfiguredProcessingUser and testFallsBackToFileOwnerIfConfiguredProcessingUserDoesNotExist.
• Updated GlobalSettingsServiceTest and GlobalSettingsControllerTest for the new setting (including null/blank handling).
• Added Vue tests for the new input in GlobalSettings.spec.js.
• Ran full PHPUnit suite against a real Nextcloud (master) checkout — all workflow_ocr tests pass except 4 pre-existing/environmental failures unrelated to this change (missing Imagick extension, running as root).
• Ran npm run test:unit (53 passed), npm run lint, and composer cs:check — all clean, no new issues introduced.

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