Convert mastodon to LAVA @artifact_processor

[abrignoni]

Splits the Mastodon parser into six decorated artifacts (category Mastodon): Hashtag Searches, Account Searches, Notifications, Timeline (DB), plus Account Details and Instance Details (JSON). Each gets its own targeted paths (db vs accounts.json vs instance*.json).

• Search time (epoch sec) and instance last_updated (epoch ms) → aware UTC datetime.
• ISO8601 created_at timestamps now parsed to aware UTC via _iso_to_utc (the original just did .replace('T',' ').replace('Z',''), leaving a naive string that would shift in LAVA) and marked datetime.
• HTML post/bio content stripped to plain text via BeautifulSoup (_strip_html); timeline attachment URLs joined.
• Fixed the accout_db typo (the original mis-spelled the variable but worked by being consistently wrong).

Verified: byte-compile, all 6 decorated 4-arg, return 3-tuples, _iso_to_utc/_strip_html unit-tested, no duplicate names repo-wide, no '&' in names, CI-style pylint 0 W/E, loader builds (439).