migration: V67 — recreate spring_session tables (ADR-020)

Re-introduces tables dropped by V2. Canonical DDL from Spring
Session 3.x schema-postgresql.sql.

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

backend/src/main/resources/db/migration/V67__recreate_spring_session_tables.sql

@@ -0,0 +1,27 @@
+-- Re-introduces the Spring Session JDBC tables that were dropped by V2 as unused.
+-- DDL copied verbatim from Spring Session 3.x schema-postgresql.sql.
+-- See ADR-020 and issue #523.
+
+CREATE TABLE spring_session (
+    PRIMARY_ID            CHAR(36) NOT NULL,
+    SESSION_ID            CHAR(36) NOT NULL,
+    CREATION_TIME         BIGINT   NOT NULL,
+    LAST_ACCESS_TIME      BIGINT   NOT NULL,
+    MAX_INACTIVE_INTERVAL INT      NOT NULL,
+    EXPIRY_TIME           BIGINT   NOT NULL,
+    PRINCIPAL_NAME        VARCHAR(100),
+    CONSTRAINT spring_session_pk PRIMARY KEY (PRIMARY_ID)
+);
+
+CREATE UNIQUE INDEX spring_session_ix1 ON spring_session (SESSION_ID);
+CREATE INDEX        spring_session_ix2 ON spring_session (EXPIRY_TIME);
+CREATE INDEX        spring_session_ix3 ON spring_session (PRINCIPAL_NAME);
+
+CREATE TABLE spring_session_attributes (
+    SESSION_PRIMARY_ID CHAR(36)     NOT NULL,
+    ATTRIBUTE_NAME     VARCHAR(200) NOT NULL,
+    ATTRIBUTE_BYTES    BYTEA        NOT NULL,
+    CONSTRAINT spring_session_attributes_pk PRIMARY KEY (SESSION_PRIMARY_ID, ATTRIBUTE_NAME),
+    CONSTRAINT spring_session_attributes_fk FOREIGN KEY (SESSION_PRIMARY_ID)
+        REFERENCES spring_session (PRIMARY_ID) ON DELETE CASCADE
+);