feat(guide): markdown-driven backend API, retire DB/CRUD (Refs #1)

Phase 1 of the guide rebuild (DESIGN.md):
- MarkdownGuideService scans content/**/*.md (frontmatter: module/title/tab/order/audience)
- GET /api/v1/guide/nav + /api/v1/guide/modules/{module}; audience gated by role
  (internal tabs only for MSP_ADMIN/BCOS_ADMIN)
- Removed JPA/Flyway/Postgres, Guide entity/repo/service, admin CRUD, seed migrations
- Seed content: devices.sw-deploy (Overview/Data/Technical-internal), fleet.tasks
- Verified: boots with no DB, loads 4 docs, nav + audience filtering + 401 all correct

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/pom.xml

@@ -29,10 +29,6 @@
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-web</artifactId>
         </dependency>
-        <dependency>
-            <groupId>org.springframework.boot</groupId>
-            <artifactId>spring-boot-starter-data-jpa</artifactId>
-        </dependency>
         <dependency>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-security</artifactId>
@@ -46,21 +42,6 @@
             <artifactId>spring-boot-starter-actuator</artifactId>
         </dependency>
 
-        <dependency>
-            <groupId>org.postgresql</groupId>
-            <artifactId>postgresql</artifactId>
-            <scope>runtime</scope>
-        </dependency>
-
-        <dependency>
-            <groupId>org.flywaydb</groupId>
-            <artifactId>flyway-core</artifactId>
-        </dependency>
-        <dependency>
-            <groupId>org.flywaydb</groupId>
-            <artifactId>flyway-database-postgresql</artifactId>
-        </dependency>
-
         <dependency>
             <groupId>com.hiveops</groupId>
             <artifactId>hiveops-security-common</artifactId>