web(posts): rename post DTOs/args away from role-suffixes; unify Create/Update result

[mdorman]

Deferred from #323 (posts vertical convergence). During #323 review we noticed the
post read/result DTOs and the #[server] arg wrappers are named for their role in
the plumbing (*Result, *Response, args) rather than for what they are — the
FooManager smell. Splitting this out so it gets its own focused review + e2e pass
rather than riding on #323's large convergence PR (which does the reorg; this does the
renaming/remodeling).

Observations

• create_post/update_post wire arg is named args ({"args": {…}} on the wire,
  from Remove too_many_arguments #[allow] from the create_post/update_post #[server] fns (#94 follow-up) #299's arg-struct fold). Generic — the endpoint already says it's the arguments.
  → name it for the noun: post ({"post": {…}}), create_post(post: CreatePostArgs).
• PostResponse is the lone read DTO named by transaction-role. Its siblings are
  DraftSummary / TimelinePostSummary (*Summary). Its own doc says "Details of a
  post." → rename to PostDetails (PostView collides with the render-twin struct).
• CreatePostResult and UpdatePostResult differ by exactly one field
  (CreatePostResult.created_at) — and created_at may not even be read by the create
  client. They're the same thing (the saved post's identity + links: post_id, slug,
  preview/permalink URLs) wearing two names. → collapse into ONE type named for what it
  is (e.g. SavedPost / PostRef / PostLinks); confirm whether created_at is used
  and drop it if not. PublishPostResult is a genuinely narrower shape (publish always
  yields a published post, so published_at/permalink are non-optional) — either
  fold it into the unified type with optionals or keep it as the one real special case.
• More broadly: audit the posts DTO family and drop *Result/*Response role-suffixes
  in favor of shape/noun names, consistent with *Summary.

Scope

• Rename the create_post/update_post wire arg args → post (updates the Rust
  dispatch sites, the TS e2e helpers, and the server integration tests that POST directly
  — same set Remove too_many_arguments #[allow] from the create_post/update_post #[server] fns (#94 follow-up) #299 touched).
• Rename PostResponse → PostDetails (also touches the shared render layer:
  PageSeed::Permalink(PostResponse) in render/mod.rs).
• Unify CreatePostResult + UpdatePostResult (and consider PublishPostResult) into one
  well-named type; drop the *Result/*Response suffixes across the posts DTOs.

Acceptance

• No posts DTO named by transaction-role (*Result/*Response); no generic args wire key.
• One saved-post result type for create/update (publish reconciled).
• cargo xtask validate green incl. the create/edit/publish/draft posts e2e (the wire
  changes are verified end-to-end).

Notes

Purely naming/modeling — no behavior change. Blocked-by #323 (do it on the converged
layout).

[mdorman]

Deferred from #329 (timeline convergence): the timeline keyset cursor is now modeled as a timeline-local TimelineCursor { created_at: UtcInstant, post_id: PostId } in web/src/timeline/state.rs. Consider giving TimelinePage an Option<TimelineCursor> on the wire (replacing the flat next_cursor_created_at/next_cursor_post_id pair) when reworking the post DTOs here, per ADR-0065's pervasive-newtype direction.

[mdorman]

Add to scope: collapse the TimelinePostSummary / PostResponse read-DTO duplication

Surfaced while relocating these DTOs to common::seed (#610 / #632). Beyond the naming/result-unification already in this issue, the two read DTOs are near-duplicates and should be consolidated as part of the posts-DTO audit:

• PostResponse is a strict superset of TimelinePostSummary: 11 fields identical (post_id, username, title, summary, slug, rendered_html, created_at, permalink, is_author, is_draft, tags); published_at differs only in optionality (UtcInstant on the always-published timeline row vs Option<UtcInstant>); PostResponse adds body+format; TimelinePostSummary has no unique field.
• They are already hand-interconverted: web/src/posts/component.rs:1225 rebuilds a TimelinePostSummary from a PostResponse field-by-field, and both collapse to a single PostView<'a> at render (web/src/posts/render.rs).
• Not duplicative of storage::PostRecord (the DB row) — both are legit view types (+ is_author/permalink, Username not UserId, TagSummary not PostTag); the duplication is between the two DTOs.

Proposed: collapse into a shared-core struct + a body/format detail extension (or one type with the published_at invariant handled). The one real design question is the always-published published_at: UtcInstant invariant on timeline rows — any merged type must preserve/encode it. Post-#610 both now live in common::seed, so the merge is co-located and cleaner.