Major restructuring

[janheinrichmerker]

• [x] Migrate from elasticsearch-dsl to (type-safe) elasticsearch-pydantic
• [x] Restructured parsing to use hard-coded (i.e., unit-testable) parsers
• [x] Migrated monitoring (Flask → FastAPI, now more flexible API)
• [x] Implement web search result block landing page download (fix #6)
• [x] Remove unused legacy code
• [x] Updated readme
• [x] Updated dependencies
• [x] Updated CI
• [x] Add dry-run option to test most CLI actions without actual writes
• [ ] Add JSONL export (and local "fake" WARC store for testing)
• [ ] Migrate "legacy" unit tests to work with the current parsing architecture