Add paginated test history listing with disable option

Add GET /tests endpoint returning lightweight test summaries (grade,
score, domain, date) with pagination, using database-level JSON
extraction to avoid loading full report blobs. The feature can be
disabled with --disable-test-list flag. Frontend includes a new
/tests/ page with table view and a conditional "History" navbar link.

Fixes: https://github.com/happyDomain/happydeliver/issues/12

api/openapi.yaml

@@ -76,6 +76,49 @@ paths:
               schema:
                 $ref: '#/components/schemas/Error'
 
+  /tests:
+    get:
+      tags:
+        - tests
+      summary: List all tests
+      description: Returns a paginated list of test summaries with scores and grades. Can be disabled via server configuration.
+      operationId: listTests
+      parameters:
+        - name: offset
+          in: query
+          schema:
+            type: integer
+            minimum: 0
+            default: 0
+          description: Number of items to skip
+        - name: limit
+          in: query
+          schema:
+            type: integer
+            minimum: 1
+            maximum: 100
+            default: 20
+          description: Maximum number of items to return
+      responses:
+        '200':
+          description: List of test summaries
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TestListResponse'
+        '403':
+          description: Test listing is disabled
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
+        '500':
+          description: Internal server error
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Error'
+
   /report/{id}:
     get:
       tags:
@@ -1365,3 +1408,53 @@ components:
           items:
             $ref: '#/components/schemas/BlacklistCheck'
           description: List of DNS whitelist check results (informational only)
+
+    TestSummary:
+      type: object
+      required:
+        - test_id
+        - score
+        - grade
+        - created_at
+      properties:
+        test_id:
+          type: string
+          pattern: '^[a-z0-9-]+$'
+          description: Test identifier (base32-encoded with hyphens)
+        score:
+          type: integer
+          minimum: 0
+          maximum: 100
+          description: Overall deliverability score (0-100)
+        grade:
+          type: string
+          enum: [A+, A, B, C, D, E, F]
+          description: Letter grade
+        from_domain:
+          type: string
+          description: Sender domain extracted from the report
+        created_at:
+          type: string
+          format: date-time
+
+    TestListResponse:
+      type: object
+      required:
+        - tests
+        - total
+        - offset
+        - limit
+      properties:
+        tests:
+          type: array
+          items:
+            $ref: '#/components/schemas/TestSummary'
+        total:
+          type: integer
+          description: Total number of tests
+        offset:
+          type: integer
+          description: Current offset
+        limit:
+          type: integer
+          description: Current limit