fix: address code review — security, thread safety, performance, and null safety

Security (CRITICAL):
- KeyUtil: read JWT signing key from DC3_SECURITY_KEY env/property
- UserPasswordServiceImpl: read default password from DC3_SECURITY_DEFAULT_PASSWORD env
- KeyLoader: read OPC-UA keystore password from OPCUA_KEYSTORE_PASSWORD env
- WebFilterConfig: return 401 on malformed X-Auth-User header
- AlgorithmConstant: document deprecated hardcoded constants

Thread safety (HIGH):
- Fix TOCTOU races in 4 driver connectors (computeIfAbsent)
- Fix PlcS7 lock leak, OpcUa connect timeout (5s)
- CoapClientManager: synchronize setURI to prevent URI race
- PointValueJob, MqttScheduleJob: add @DisallowConcurrentExecution
- SystemHealthServiceImpl: preserve interrupt flag
- WindowSampleBuffer: local AtomicInteger → int

Performance (MEDIUM):
- EntityStateExpiryScanner: batch alarm saves (saveBatch)
- ImportDeviceServiceImpl: batch config saves
- ResourceRegistrySyncServiceImpl: batch-load nodes, eliminate N+1 COUNTs
- DriverSenderServiceImpl: debug-gate hot-path logging
- PointServiceImpl: stream().count() → size()

Null safety (LOW):
- RegexUtil: null guards on isName/isPhone/isMail/isPassword/isHost
- HostUtil: null-check getNetworkInterfaces() return
- TimeUtil: log parse failures instead of silent null

Infrastructure:
- Add spring-boot-starter-cache + @EnableCaching CacheConfig
- Replace embedded modbus4j/plc-s7 jars with external Maven dependencies
- Add dc3-driver-modbus-rtu module

pom.xml

@@ -77,8 +77,26 @@
         <mockito.javaagent.argLine>
             -javaagent:${settings.localRepository}/org/mockito/mockito-core/${mockito.version}/mockito-core-${mockito.version}.jar
         </mockito.javaagent.argLine>
+
+        <!-- Modbus -->
+        <modbus4j.version>3.0.3</modbus4j.version>
+        <jserialcomm.version>2.11.0</jserialcomm.version>
     </properties>
 
+    <repositories>
+        <repository>
+            <releases>
+                <enabled>true</enabled>
+            </releases>
+            <snapshots>
+                <enabled>false</enabled>
+            </snapshots>
+            <id>ias-releases</id>
+            <name>Infinite Automation Release Repository</name>
+            <url>https://maven.mangoautomation.net/repository/ias-release/</url>
+        </repository>
+    </repositories>
+
     <modules>
         <module>dc3-api</module>
         <module>dc3-common</module>
@@ -255,6 +273,20 @@
                 <artifactId>dc3-common-web</artifactId>
                 <version>${dc3.version}</version>
             </dependency>
+
+            <!-- Modbus Related -->
+            <dependency>
+                <groupId>com.infiniteautomation</groupId>
+                <artifactId>modbus4j</artifactId>
+                <version>${modbus4j.version}</version>
+            </dependency>
+
+            <!-- Serial Communication -->
+            <dependency>
+                <groupId>com.fazecast</groupId>
+                <artifactId>jSerialComm</artifactId>
+                <version>${jserialcomm.version}</version>
+            </dependency>
         </dependencies>
     </dependencyManagement>