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/auth/users.go

@@ -2,10 +2,16 @@ package auth
 
 import (
 	"database/sql"
+	"errors"
 	"fmt"
 	"time"
 )
 
+// dummyHash is a pre-computed Argon2id hash used for constant-time comparison
+// when a user is not found. This prevents timing attacks that reveal whether
+// a username exists.
+var dummyHash = "$argon2id$v=19$m=65536,t=3,p=4$AAAAAAAAAAAAAAAAAAAAAA$AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+
 // CreateUser creates a new user with a hashed password.
 func CreateUser(database *sql.DB, username, password string, params Argon2Params) (int64, error) {
 	hash, err := HashPassword(password, params)
@@ -32,16 +38,22 @@ func AuthenticateUser(database *sql.DB, username, password string) (int64, error
 	err := database.QueryRow(
 		"SELECT id, password_hash FROM users WHERE username = ?", username,
 	).Scan(&userID, &hash)
+	if errors.Is(err, sql.ErrNoRows) {
+		// User not found: verify against dummy hash to consume constant time,
+		// preventing timing attacks that reveal username existence.
+		_, _ = VerifyPassword(password, dummyHash)
+		return 0, fmt.Errorf("invalid credentials")
+	}
 	if err != nil {
-		return 0, fmt.Errorf("user not found")
+		return 0, fmt.Errorf("invalid credentials")
 	}
 
 	ok, err := VerifyPassword(password, hash)
 	if err != nil {
-		return 0, err
+		return 0, fmt.Errorf("invalid credentials")
 	}
 	if !ok {
-		return 0, fmt.Errorf("invalid password")
+		return 0, fmt.Errorf("invalid credentials")
 	}
 
 	return userID, nil