Convert simple/medium importers to BaseImporter classes

[aquarion]

Convert the remaining straightforward imports/ scripts to class-based importers using BaseImporter in src/lifestream/importers/.

These have API key or simple login auth — no OAuth flows to untangle.

Files

File	Auth	Notes
imports/ffxiv.py	API key (xivapi)	Already has good structure
imports/atproto_posts.py	username/password login	Already refactored in #62
imports/instagram.py	username/password login	Uses unofficial InstagramAPI
imports/gw2.py	API key	Already has run_import()/main() structure
imports/oyster.py	None (mechanize scraping)	Takes args from CLI
imports/oyster_csv.py	None (CSV)	Trivial

Pattern

Each conversion should:

1. Create src/lifestream/importers/<name>.py with a class XImporter(BaseImporter)
2. Move config loading to get_config() calls
3. Replace sys.exit() with ConfigurationError
4. Add a main() entry point and wire up pyproject.toml
5. Delete the old imports/<name>.py