Security hardening: fix critical, high, and medium issues from audit

CRITICAL:
- A-001: SQL injection in snapshot — escape single quotes in backup path
- A-002: Timing attack — always verify against dummy hash when user not
  found, preventing username enumeration
- A-003: Notebook ownership — all authenticated endpoints now verify
  user_id before loading notebook data
- A-004: Point data bounds — decodePoints returns error on misaligned
  data, >4MB payloads, and NaN/Inf values

HIGH:
- A-005: Error messages — generic errors in HTTP responses, no err.Error()
- A-006: Share link authz — RevokeShareLink verifies notebook ownership
- A-007: Scan errors — return 500 instead of silently continuing

MEDIUM:
- A-008: Web server TLS — optional TLS support (HTTPS when configured)
- A-009: Input validation — page_size, stroke count, point_data alignment
  checked in SyncNotebook RPC
- A-010: Graceful shutdown — 30s drain on SIGINT/SIGTERM, all servers
  shut down properly

Added AUDIT.md with all 17 findings, status, and rationale for
accepted risks.

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

internal/render/jpg.go

@@ -22,7 +22,10 @@ func RenderJPG(pageSize string, strokes []Stroke, quality int) ([]byte, error) {
 
 	// Draw strokes
 	for _, s := range strokes {
-		points := decodePoints(s.PointData)
+		points, err := decodePoints(s.PointData)
+		if err != nil {
+			continue
+		}
 		if len(points) < 4 {
 			continue
 		}