Initialize eng-pad-server with project documentation

- README.md: project overview, quick start, build commands
- CLAUDE.md: AI dev context, source tree, key conventions
- ARCHITECTURE.md: full system spec covering data model, auth
  (password + FIDO2/U2F), gRPC sync API, REST API, SVG/JPG/PDF
  rendering, web UI, configuration, deployment, security
- PROJECT_PLAN.md: 11 phases with discrete checkboxable steps
- PROGRESS.md: decision log and completion tracking

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PROGRESS.md

@@ -0,0 +1,27 @@
+# PROGRESS.md — eng-pad-server Implementation Progress
+
+This file tracks completed work and decisions. Updated after every step.
+See PROJECT_PLAN.md for the full step list.
+
+## Completed
+
+_(none yet)_
+
+## In Progress
+
+Phase 0: Project Setup
+
+## Decisions
+
+- **Language**: Go (Metacircular standard)
+- **Database**: SQLite via `modernc.org/sqlite` (pure Go, no CGo)
+- **Auth**: Argon2id passwords + FIDO2/U2F via `go-webauthn/webauthn`
+- **gRPC auth**: username/password in metadata per-request (no tokens)
+- **Web auth**: password → bearer token in session cookie
+- **Rendering**: SVG for web viewing, JPG/PDF for export
+- **Sync model**: full notebook replacement (upsert), no incremental sync
+- **Share links**: 32-byte random token, optional expiry, scoped to notebook
+- **Grid**: not rendered server-side (tablet writing aid only)
+- **Coordinate system**: 300 DPI canonical, scaled to 72 DPI for SVG/PDF
+- **FIDO2/U2F**: web UI login only, not gRPC sync
+- **Server is read-only**: mirrors tablet exactly, no content modification