Scaffold btest-server-pro: multi-user, quotas, LDAP

New binary `btest-server-pro` (build with --features pro):
  cargo build --release --features pro --bin btest-server-pro

Modules:
- server_pro/user_db.rs: SQLite user database with usage tracking
  - Users table (username, password_hash, quotas, enabled)
  - Usage table (daily bytes per user)
  - Sessions table (per-connection tracking)
- server_pro/quota.rs: bandwidth quota enforcement
  - Per-user daily/weekly limits
  - Per-IP connection limits
  - Max test duration
- server_pro/ldap_auth.rs: LDAP/AD authentication via ldap3
  - Simple bind authentication
  - Service account search for user DN

CLI flags: --users-db, --ldap-url, --ldap-base-dn, --ldap-bind-dn,
  --ldap-bind-pass, --daily-quota, --weekly-quota, --max-conn-per-ip,
  --max-duration

Binary sizes: btest=1.8MB, btest-server-pro=3.4MB (SQLite bundled)
Standard btest binary unchanged, 58 tests pass.

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

Cargo.toml

@@ -16,6 +16,15 @@ path = "src/lib.rs"
 name = "btest"
 path = "src/main.rs"
 
+[[bin]]
+name = "btest-server-pro"
+path = "src/server_pro/main.rs"
+required-features = ["pro"]
+
+[features]
+default = []
+pro = ["dep:rusqlite", "dep:ldap3"]
+
 [dependencies]
 tokio = { version = "1", features = ["full"] }
 clap = { version = "4", features = ["derive"] }
@@ -32,6 +41,8 @@ num-traits = "0.2.19"
 num-integer = "0.1.46"
 sha2 = "0.11.0"
 hostname = "0.4.2"
+rusqlite = { version = "0.39.0", features = ["bundled"], optional = true }
+ldap3 = { version = "0.12.1", optional = true }
 
 [profile.release]
 opt-level = 3