refactor: remove unused Spring Session JDBC dependency and tables

Spring Session was pulled in as a dependency but never used — auth is
stateless HTTP Basic, so sessions are never written. Removed:
- spring-boot-starter-session-jdbc (and test variant) from pom.xml
- spring_session and spring_session_attributes tables/indexes/constraints
  from V1__initial_schema.sql

Added V2 migration to drop the tables on existing databases that already
ran V1.

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

backend/pom.xml

@@ -50,10 +50,6 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-security</artifactId>
 		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-session-jdbc</artifactId>
-		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-webmvc</artifactId>
@@ -98,11 +94,6 @@
 			<artifactId>spring-boot-starter-security-test</artifactId>
 			<scope>test</scope>
 		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-session-jdbc-test</artifactId>
-			<scope>test</scope>
-		</dependency>
 		<dependency>
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-webmvc-test</artifactId>