Refactor pagination with cursor-based support and standardized response format

- Implement cursor-based pagination in pagination.py
- Update response models to standardize pagination structure
- Add cursor pagination utilities for repositories
- Update HistoryArchiveRepository and ImportLogRepository with new pagination
- Add comprehensive tests for cursor pagination
- Update documentation for backend development and task tracking

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/db.py

@@ -107,7 +107,7 @@ _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_HISTORY_ARCHIVE,
 ]
 
-_CURRENT_SCHEMA_VERSION: int = 6
+_CURRENT_SCHEMA_VERSION: int = 7
 
 _MIGRATIONS: dict[int, str] = {
     1: "\n".join(_SCHEMA_STATEMENTS),
@@ -187,10 +187,25 @@ CREATE TABLE IF NOT EXISTS import_runs (
 -- Index for looking up completed imports by source
 CREATE INDEX IF NOT EXISTS idx_import_runs_source_status
     ON import_runs (source_id, status);
+""",
+    7: """
+-- Migration 7: Add indexes to import_log table for cursor-based pagination.
+-- The import_log table is paginated by id (newest first) and filtered by source_id.
+-- These indexes accelerate pagination queries and maintain consistent ordering.
+-- See Docs/Backend-Development.md § Database Performance for details.
+
+-- Index for ordering by id DESC for cursor-based pagination (newest first)
+CREATE INDEX IF NOT EXISTS idx_import_log_id_desc
+    ON import_log (id DESC);
+
+-- Composite index for source_id + id DESC ordering (filtered pagination)
+CREATE INDEX IF NOT EXISTS idx_import_log_source_id_desc
+    ON import_log (source_id, id DESC);
 """,
 }
 
 
+
 # ---------------------------------------------------------------------------
 # Public API
 # ---------------------------------------------------------------------------