feat: update check (#561)

feat(update-check): add robust update check with interval support, state management, and CLI integration

- Implement version and interval-based update checks with configurable settings
- Add CLI command `kleinanzeigen-bot update-check` for manual checks
- Introduce state file with versioning, UTC timestamps, and migration logic
- Validate and normalize intervals (1d–4w) with fallback for invalid values
- Ensure correct handling of timezones and elapsed checks
- Improve error handling, logging, and internationalization (i18n)
- Add comprehensive test coverage for config, interval logic, migration, and CLI
- Align default config, translations, and schema with new functionality
- Improve help command UX by avoiding config/log loading for `--help`
- Update documentation and README with full feature overview

docs/update-check.md

@@ -0,0 +1,96 @@
+# Update Check Feature
+
+## Overview
+
+The update check feature automatically checks for newer versions of the bot on GitHub. It supports two channels:
+- `latest`: Only final releases
+- `prerelease`: Includes pre-releases
+
+## Configuration
+
+```yaml
+update_check:
+  enabled: true  # Enable/disable update checks
+  channel: latest  # One of: latest, prerelease
+  interval: 7d    # Check interval (e.g. 7d for 7 days)
+```
+
+### Interval Format
+
+The interval is specified as a number followed by a unit:
+- `s`: seconds
+- `m`: minutes
+- `h`: hours
+- `d`: days
+- `w`: weeks
+
+Examples:
+- `7d`: Check every 7 days
+- `12h`: Check every 12 hours
+- `1w`: Check every week
+
+Validation rules:
+- Minimum interval: 1 day (`1d`)
+- Maximum interval: 4 weeks (`4w`)
+- Value must be positive
+- Only supported units are allowed
+
+## State File
+
+The update check state is stored in `.temp/update_check_state.json`. The file format is:
+
+```json
+{
+  "version": 1,
+  "last_check": "2024-03-20T12:00:00+00:00"
+}
+```
+
+### Fields
+
+- `version`: Current state file format version (integer)
+- `last_check`: ISO 8601 timestamp of the last check (UTC)
+
+### Migration
+
+The state file supports version migration:
+- Version 0 to 1: Added version field
+- Future versions will be migrated automatically
+
+### Timezone Handling
+
+All timestamps are stored in UTC:
+- When loading:
+  - Timestamps without timezone are assumed to be UTC
+  - Timestamps with timezone are converted to UTC
+- When saving:
+  - All timestamps are converted to UTC before saving
+  - Timezone information is preserved in ISO 8601 format
+
+### Edge Cases
+
+The following edge cases are handled:
+- Missing state file: Creates new state file
+- Corrupted state file: Creates new state file
+- Invalid timestamp format: Logs warning, uses current time
+- Permission errors: Logs warning, continues without saving
+- Invalid interval format: Logs warning, performs check
+- Interval too short/long: Logs warning, performs check
+
+## Error Handling
+
+The update check feature handles various error scenarios:
+- Network errors: Logs error, continues without check
+- GitHub API errors: Logs error, continues without check
+- Version parsing errors: Logs error, continues without check
+- State file errors: Logs error, creates new state file
+- Permission errors: Logs error, continues without saving
+
+## Logging
+
+The feature logs various events:
+- Check results (new version available, up to date, etc.)
+- State file operations (load, save, migration)
+- Error conditions (network, API, parsing, etc.)
+- Interval validation warnings
+- Timezone conversion information