feat: auto-mark documents as completed on reaching last page

[specbug]

Summary

Adds reading completion as a first-class state. Finished docs stop blending into the Reading list and pick up a quiet visual marker everywhere they appear, without changing any reading behavior (you can still open, scroll, annotate, and review them as before).

• Backend stamps PDFFile.completed_at the first time PATCH /files/{id}/position reports last_read_position >= total_pages - 1. Auto-only — never cleared, never re-triggered. Silently skipped when total_pages is unknown.
• LibraryScreen splits the list into READING (current sort applies) and COMPLETED (always sorted by completedAt desc). Completed docs end up at the bottom but in a self-contained, named section.
• DocGlyph inverts to a sealed-stamp variant when doc.completed === true: solid ink fill, paper-colored starburst, small grayscale tick inset bottom-right. Per DESIGN.md §3, completion is not an accent moment, so the whole change is grayscale.
• PdfScreen topbar adds a mono FINISHED · YYYY·MM·DD tag next to the author whenever a doc is completed. The position-save promise patches fileMetadata.completed_at from the response, so the tag flips in place the instant the user reaches the last page — no refresh required.

Schema + migration

• New column: pdf_files.completed_at DATETIME NULL.
• In-app idempotent migration _migrate_pdf_file_completed_at runs on every backend boot (mirrors _migrate_pdf_file_title).
• Standalone migration script: apps/api/scripts/migrations/migrate_add_completed_at.py for explicit runs against deployed databases.
• PDFFileResponse.completed_at exposed; adapters.toLibraryDoc projects it as { completed, completedAt }.

Docs

• CLAUDE.md — new capability bullet, Database Schema paragraph for completed_at, current-branch block refreshed.
• apps/webapp/DESIGN.md — DocGlyph entry gets a "Completed variant" subsection; per-screen intent table notes Library sectioning and the PdfScreen FINISHED tag.

Test plan

• FastAPI TestClient end-to-end against a forked database:
  • page 3/5 → completed_at stays None
  • page 4/5 (last) → completed_at stamped
  • scroll back to page 1 → completed_at preserved
  • row with total_pages = NULL at position 999 → completed_at stays None
• Standalone migration script run on a real-data DB fork — adds column, leaves existing rows with NULL.
• bun run build compiles (one pre-existing ESLint warning in PdfScreen.js:1027 for scrollToPageIndex dep, unrelated to this change).
• Visual smoke in browser with a seeded library: Library shows READING · N + COMPLETED · N sections; completed glyphs render the sealed-stamp variant; opening a completed doc shows the FINISHED · … topbar tag.

Co-Authored-By: Claude noreply@anthropic.com