Add database migration 5: Indexes for history_archive query performance

- Add composite index on (jail, timeofban DESC) for dashboard filtering
- Add composite index on (timeofban DESC, jail, action) for time-range queries
- Add single-column indexes on ip and action for targeted filtering
- Update schema version to 5 and document in Backend-Development.md

Indexes optimize common dashboard and API query patterns with pagination.

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 = 4
+_CURRENT_SCHEMA_VERSION: int = 5
 
 _MIGRATIONS: dict[int, str] = {
     1: "\n".join(_SCHEMA_STATEMENTS),
@@ -143,6 +143,29 @@ CREATE TABLE scheduler_lock (
     created_at REAL NOT NULL,
     heartbeat_at REAL NOT NULL
 );
+""",
+    5: """
+-- Migration 5: Add indexes to history_archive table for query performance.
+-- The history_archive table supports filtering by jail, IP, action, and time range,
+-- combined with pagination (ORDER BY timeofban DESC LIMIT/OFFSET).
+-- These indexes accelerate common dashboard and API queries.
+-- See Docs/Backend-Development.md § Database Performance for details.
+
+-- Composite index for common queries: jail + timeofban ordering (dashboard filter).
+CREATE INDEX IF NOT EXISTS idx_history_archive_jail_timeofban
+    ON history_archive (jail, timeofban DESC);
+
+-- Composite index for time-range + jail queries (history timeline filters).
+CREATE INDEX IF NOT EXISTS idx_history_archive_timeofban_jail_action
+    ON history_archive (timeofban DESC, jail, action);
+
+-- Index for single-column filters: supports IP prefix searches and exact matches.
+CREATE INDEX IF NOT EXISTS idx_history_archive_ip
+    ON history_archive (ip);
+
+-- Index for action-based queries: supports ban/unban filtering.
+CREATE INDEX IF NOT EXISTS idx_history_archive_action
+    ON history_archive (action);
 """,
 }