Replace panicking From impls with TryFrom for untrusted export/API data

[lmorchard]

Found during a code-review pass (2026-07-23). Related to #14 and #35.

Several infallible From conversions .unwrap() on untrusted export/API JSON, so one malformed record panics the whole import/fetch instead of being skipped or reported.

• impl From<serde_json::Value> for Actor → serde_json::from_value(value).unwrap() (src/activitystreams.rs), invoked on export data in src/mastodon/importer.rs. Actor has many non-optional fields; any missing/renamed field or bad published timestamp aborts import.
• impl From<Status> for Activity → url::Url::parse(status.uri).unwrap() (src/activitystreams.rs), on remote API responses in src/mastodon/fetcher.rs.
• Related panics in the fetch hot path: access_token...unwrap() (panics when running fetch before mastodon link — see Bug: Crash when attempting mastodon fetch against new instance, because actors haven't been populated #35), actors.get(actor_id).unwrap(), activity.actor.id().unwrap() in src/mastodon/fetcher.rs, and unwrap()s over unresolved actors in src/site_generator.rs.

Suggested fix

Replace the infallible From impls with TryFrom (or call serde_json::from_value/Url::parse at the call site with ?) so callers can skip-with-warning or report bad records. Turn the token/actor lookups into checked errors with actionable messages.

Severity: high (crash-on-malformed-input across the two main ingest paths).