test(#120): add JaCoCo branch coverage gate to Maven build

Adds JaCoCo 0.8.12 with prepare-agent, report, and check executions.
Baseline measured at 46.8% branch coverage. Gate set at 42% (baseline
minus 5%) to prevent regression while giving room to close the gap.

Excluded from measurement: DTOs, config classes, model entities,
ErrorCode enum — these contain no testable branch logic.

Target is 80%; gap documented in issue #120.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pom.xml

@@ -171,6 +171,51 @@
 
 	<build>
 		<plugins>
+			<plugin>
+				<groupId>org.jacoco</groupId>
+				<artifactId>jacoco-maven-plugin</artifactId>
+				<version>0.8.12</version>
+				<configuration>
+					<excludes>
+						<exclude>**/dto/**</exclude>
+						<exclude>**/config/**</exclude>
+						<exclude>**/exception/ErrorCode*</exclude>
+						<exclude>**/model/**</exclude>
+					</excludes>
+				</configuration>
+				<executions>
+					<execution>
+						<id>prepare-agent</id>
+						<goals><goal>prepare-agent</goal></goals>
+					</execution>
+					<execution>
+						<id>report</id>
+						<phase>verify</phase>
+						<goals><goal>report</goal></goals>
+					</execution>
+					<!-- Gate: current baseline 46.8% — threshold set at 42% to prevent regression.
+					     Target is 80%; close the gap by adding tests for service error paths. -->
+					<execution>
+						<id>check</id>
+						<phase>verify</phase>
+						<goals><goal>check</goal></goals>
+						<configuration>
+							<rules>
+								<rule>
+									<element>BUNDLE</element>
+									<limits>
+										<limit>
+											<counter>BRANCH</counter>
+											<value>COVEREDRATIO</value>
+											<minimum>0.42</minimum>
+										</limit>
+									</limits>
+								</rule>
+							</rules>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
 			<plugin>
 				<groupId>org.springframework.boot</groupId>
 				<artifactId>spring-boot-maven-plugin</artifactId>