feat: show live build version in web UI

- /api/version returns version, commitSha, buildDate, trashRetentionDays
- CI passes COMMIT_SHA (full), BUILD_DATE and VERSION (git tag) as
  docker build args; Dockerfile bakes them as env
- sidebar footer shows version/sha linked to the Gitea commit
- admin System tab shows version, commit link, build date, trash retention

lib/api-spec/openapi.yaml

@@ -37,6 +37,20 @@ paths:
               schema:
                 $ref: "#/components/schemas/HealthStatus"
 
+  /version:
+    get:
+      operationId: getVersion
+      tags: [health]
+      summary: Build version information
+      description: Returns the running build version, commit SHA and build date
+      responses:
+        "200":
+          description: Version information
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/VersionInfo"
+
   /tools:
     get:
       operationId: listTools
@@ -660,6 +674,19 @@ components:
       required:
         - status
 
+    VersionInfo:
+      type: object
+      required: [version]
+      properties:
+        version:
+          type: string
+        commitSha:
+          type: ["string", "null"]
+        buildDate:
+          type: ["string", "null"]
+        trashRetentionDays:
+          type: integer
+
     AuthMode:
       type: object
       required: [mode]