feat(web): render GitHub assignees on stories

[Nxssie]

Summary

Closes #101.

Facility already mirrors gh_issues.assignees end to end — webhook sync,
services/api/src/pipeline.ts, and both PipelineStorySchema.assignees and
StoryDetailSchema.assignees — but apps/web never read the field, so
ownership was invisible without opening GitHub.

• Render @login assignee chips on the story row (IssueRow) and the story
  detail header, in the existing label/timestamp grammar. Several assignees
  collapse to the first plus +N; unassigned renders nothing (Backlog's
  common case), not a placeholder.
• Add AssigneeChip: GitHub avatar (github.com/{login}.png, no new sync
  column needed) with an initial-letter fallback for deployments where the
  browser can't reach github.com.
• Add a "mine" filter on the Stories board, next to the stage chips, matching
  against the signed-in user's githubLogin (from Show the signed-in GitHub user, and assign them on the issue and the PR #70).

Test plan

• bunx tsc --noEmit — clean
• bunx vitest run (apps/web, full suite) — 67/67 passing, including new
  coverage in pipeline-story.test.ts for the unassigned, +N, and
  avatar/fallback cases
• bunx biome check on touched files — clean
• Manual check in a browser (not run in this environment)

[Nxssie]

Pushed a follow-up commit (a98dd21) after manually walking through this on a local instance:

• The mine filter was originally placed inline between "all" and the stage tabs. Visually that read as if it were part of the same single-select group ("all", "Shipped", "In review", ...), when it's actually an independent axis (assignee, not stage) — the two can be active at once (e.g. "all stages, only mine").
• Moved it to the end of the filter row, set off with a vertical divider, so it reads as its own filter rather than a peer of the stage tabs.

No behavior change to the filtering logic itself, just the grouping/placement.