feat(lemmyv1): surface read_comments_at on PostView

[aeharding]

What

Surface read_comments_at as an optional field on PostView.

Lemmy v1 records the timestamp of the last time the current user read a post's comments in post_actions.read_comments_at. Until now the compat layer only consumed it inside toUnreadComments (to derive unread_comments) and discarded the raw value.

// schemas/PostView.ts
read_comments_at: z.optional(z.string()),

// providers/lemmyv1/compat.ts (toPostView)
read_comments_at: nullToUndef(v.post_actions?.read_comments_at),

Why

It unlocks two client features (e.g. Voyager) that the count alone can't:

• Distinguish never-opened posts from opened-with-new-comments. toUnreadComments returns the total comment count when read_comments_at is unset, so unread_comments > 0 alone can't tell "5 brand-new comments since I last read" from "5 comments, never opened". The raw timestamp lets clients gate the "N new" badge on read_comments_at != null.
• Highlight individual unread comments — comments whose published_at is newer than read_comments_at.

Provider coverage

• Lemmy v1 — populated from post_actions.read_comments_at.
• Lemmy v0 / PieFed — left undefined; neither exposes an equivalent read-comments timestamp.

Notes

• Normalized via nullToUndef: v1 sends JSON null (not an omitted key) for unset fields on an existing post_actions row, and the schema is z.optional(z.string()) (no null). This matches the existing read_at / vote / notification-id handling in the same file — toUnreadComments already guards read_comments_at == null for the same reason.

Verification

• pnpm test (lint + types + vitest): 25 passed, including 2 new lemmyv1-compat cases — surfaces the timestamp, and maps null/absent → undefined.
• Confirmed against a live 1.0.0-nightly instance: get_post returns the pre-update read_comments_at (the boundary) and /post/list carries post_actions.read_comments_at per post.