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>

backend/src/main/resources/db/migration/V1__initial_schema.sql

@@ -66,32 +66,6 @@ CREATE TABLE public.persons (
 );
 
 
---
--- Name: spring_session; Type: TABLE; Schema: public; Owner: -
---
-
-CREATE TABLE public.spring_session (
-    primary_id character(36) NOT NULL,
-    session_id character(36) NOT NULL,
-    creation_time bigint NOT NULL,
-    last_access_time bigint NOT NULL,
-    max_inactive_interval integer NOT NULL,
-    expiry_time bigint NOT NULL,
-    principal_name character varying(100)
-);
-
-
---
--- Name: spring_session_attributes; Type: TABLE; Schema: public; Owner: -
---
-
-CREATE TABLE public.spring_session_attributes (
-    session_primary_id character(36) NOT NULL,
-    attribute_name character varying(200) NOT NULL,
-    attribute_bytes bytea NOT NULL
-);
-
-
 --
 -- Name: tag; Type: TABLE; Schema: public; Owner: -
 --
@@ -168,22 +142,6 @@ ALTER TABLE ONLY public.persons
     ADD CONSTRAINT persons_pkey PRIMARY KEY (id);
 
 
---
--- Name: spring_session_attributes spring_session_attributes_pk; Type: CONSTRAINT; Schema: public; Owner: -
---
-
-ALTER TABLE ONLY public.spring_session_attributes
-    ADD CONSTRAINT spring_session_attributes_pk PRIMARY KEY (session_primary_id, attribute_name);
-
-
---
--- Name: spring_session spring_session_pk; Type: CONSTRAINT; Schema: public; Owner: -
---
-
-ALTER TABLE ONLY public.spring_session
-    ADD CONSTRAINT spring_session_pk PRIMARY KEY (primary_id);
-
-
 --
 -- Name: tag tag_name_key; Type: CONSTRAINT; Schema: public; Owner: -
 --
@@ -240,27 +198,6 @@ ALTER TABLE ONLY public.users
     ADD CONSTRAINT users_username_key UNIQUE (username);
 
 
---
--- Name: spring_session_ix1; Type: INDEX; Schema: public; Owner: -
---
-
-CREATE UNIQUE INDEX spring_session_ix1 ON public.spring_session USING btree (session_id);
-
-
---
--- Name: spring_session_ix2; Type: INDEX; Schema: public; Owner: -
---
-
-CREATE INDEX spring_session_ix2 ON public.spring_session USING btree (expiry_time);
-
-
---
--- Name: spring_session_ix3; Type: INDEX; Schema: public; Owner: -
---
-
-CREATE INDEX spring_session_ix3 ON public.spring_session USING btree (principal_name);
-
-
 --
 -- Name: users_groups fk1qwqr38oun4xcttk576wevhxn; Type: FK CONSTRAINT; Schema: public; Owner: -
 --
@@ -325,10 +262,4 @@ ALTER TABLE ONLY public.document_receivers
     ADD CONSTRAINT fks7t60twjgfmpeqcuc3g0fvjpm FOREIGN KEY (document_id) REFERENCES public.documents(id);
 
 
---
--- Name: spring_session_attributes spring_session_attributes_fk; Type: FK CONSTRAINT; Schema: public; Owner: -
---
-
-ALTER TABLE ONLY public.spring_session_attributes
-    ADD CONSTRAINT spring_session_attributes_fk FOREIGN KEY (session_primary_id) REFERENCES public.spring_session(primary_id) ON DELETE CASCADE;
 

backend/src/main/resources/db/migration/V2__drop_spring_session_tables.sql

@@ -0,0 +1,9 @@
+-- Spring Session JDBC was included as a dependency but never used.
+-- Authentication is stateless HTTP Basic Auth; sessions are never written.
+-- These tables were created by V1 and are safe to remove.
+
+ALTER TABLE IF EXISTS public.spring_session_attributes
+    DROP CONSTRAINT IF EXISTS spring_session_attributes_fk;
+
+DROP TABLE IF EXISTS public.spring_session_attributes;
+DROP TABLE IF EXISTS public.spring_session;