feat: implement structured error handling and logging

[emmanuelmingo]

Introduces a custom exception hierarchy, structured rotating file-based logging, standardised API error responses, and proper error tracking for asynchronous Celery tasks.

Changes

exceptions.py (new)

• WikifileTransferError as root base for all application exceptions
• APIError(HTTPException) as base for HTTP-facing errors; subclasses: ValidationError (422), NotFoundError (404), AuthenticationError (401), UploadError (502), ExternalAPIError (502)
• Internal exceptions: DownloadError, DatabaseError, TaskError

logging_config.py (new)

• configure_logging() sets up a RotatingFileHandler (5 MB / 3 backups) writing to logs/app.log plus a console handler
• Extracted from app.py so logging config lives in its own module

error_handlers.py (updated)

• register_error_handlers() now logs HTTP errors at WARNING and unexpected exceptions at ERROR with full traceback
• All responses follow a consistent envelope: {"success": bool, "data": {}, "errors": [...]}

app.py (updated)

• Import and call configure_logging() from logging_config
• Raise ValidationError / AuthenticationError / ExternalAPIError / DownloadError / DatabaseError at every failure point instead of returning manual JSON error responses
• /api/task_status: fix PROGRESS state data (use task.info) and SUCCESS state data (use task.result); remove non-serialisable result
• Replace bare except: clauses with specific exception types
• Standardise response envelope to use "errors" list throughout

utils.py (updated)

• download_image: raise DownloadError instead of returning None; specific messages for network errors, missing imageinfo, disk failures
• process_upload: raise UploadError instead of returning None; specific messages for CSRF, network, file-read, and wiki-rejection
• get_localized_wikitext: replace bare except with RequestException and (KeyError, IndexError) handlers; log at appropriate levels
• All file handles opened with context managers (with open(...))

tasks.py (updated)

• Add structured logging with task_id on every log line
• Wrap each stage (CSRF fetch, file upload, response parse) in try/except raising TaskError so Celery marks the task FAILURE
• File opened with context manager to prevent handle leaks
• Remove unused import os

tests/ (new)

• tests/conftest.py: session-scoped Flask test client with MWOAuth and Celery mocked out; auth_client fixture for authenticated routes
• tests/test_exceptions.py: 33 tests covering inheritance chain, HTTP status codes, werkzeug compatibility, and message handling