feat: M4 chained-cron checkpoint generation + board button (v0.21.0)

## Board 'New Article' button
- templates/admin/board-page.php: add #prab-board-generate-now button + status div
- includes/admin/class-board-page.php: inject generateNonce + i18n strings into prabBoard;
  enqueue companion board-generate.js
- assets/js/board-generate.js: NEW – board button AJAX handler (fires
  prautoblogger_generate_now, resets board backoff on success)
- assets/js/board.js: applySnapshot() re-enables button when no active runs

## Chained-cron checkpoint machine (retires SSH workaround)
- includes/core/class-generation-checkpoint-runner.php: NEW (295 lines)
  Tick 1 (prautoblogger_gen_orchestrate): orchestrate_only() → persist queue → schedule Tick 2
  Tick 2..N (prautoblogger_gen_tick): pop one idea → Article_Worker → reschedule or finalize
  Budget/halt contract + lock release on all exit paths
- includes/class-executor.php: on_manual_generation() replaced with 1-line delegate to
  Generation_Checkpoint_Runner::on_orchestrate_tick() (SSH workaround retired)
- includes/class-prautoblogger.php: register ORCHESTRATE_ACTION + GENERATE_ACTION hooks
- includes/core/class-pipeline-runner.php: expose orchestrate_only() public wrapper
- includes/core/class-pipeline-status.php: add write_initial() + write_error() helpers

## WP-CLI command
- includes/admin/class-wp-cli-commands.php: wp prautoblogger generate → kick_off()

## IA / perf pass (settings-backed pagination + DB indexes)
- includes/admin/class-board-data-provider.php: replace hardcoded posts_per_page=20 with
  get_option('prautoblogger_board_column_limit', DEFAULT)
- includes/admin/class-board-gen-log-query.php: replace LIMIT 10/20 literals with settings-backed values
- includes/admin/class-ideas-browser-query.php: NEW – query helpers extracted from Ideas_Browser
  (300-line rule split): query_ideas(), get_type_counts(), load_idea_data(),
  find_post_by_run_id(), update_idea_stage()
- includes/admin/class-ideas-browser.php: delegates all query work to Ideas_Browser_Query
- includes/class-activator.php: set_default_options() adds board_column_limit + ideas_per_page
- includes/admin/class-admin-page.php: add new settings to numeric sanitization list
- prautoblogger.php: PRAUTOBLOGGER_DEFAULT_BOARD_COLUMN_LIMIT=20, PRAUTOBLOGGER_DEFAULT_IDEAS_PER_PAGE=30
- includes/class-schema-installer.php: indexes on generation_log + analysis_results (db 1.4.0)
- includes/class-pipeline-schema-installer.php: KEY started_at on runs table

## Guard P2s
- scripts/check-constants.php: --self-test mode + inline CONSTANTS_GUARD_WINDOW comment
- .github/workflows/ci.yml: check-constants-self-test step after main constants check

## PHPUnit test
- tests/unit/Core/GenerationCheckpointRunnerTest.php: NEW (227 lines)
  4 tests: kick_off schedules+transient, empty-queue finalizes, halted-run aborts+cleanup,
  multi-idea pops-one-and-reschedules

## Docs + version
- prautoblogger.php: Version 0.21.0, PRAUTOBLOGGER_DB_VERSION 1.4.0
- ARCHITECTURE.md: §25 Chained-Cron Checkpoint Generation Path
- CHANGELOG.md: v0.21.0 entry
- INFORMATION-ARCHITECTURE.md: NEW – admin pages, entry points, DB tables, options, CLI
- uninstall.php: purge prautoblogger_gen_orchestrate + prautoblogger_gen_tick cron hooks

Agent-Session: phase2-admin-m4-20260614

assets/js/board.js

@@ -164,6 +164,11 @@
 		renderColumn( 'in_review',  snapshot.in_review   || [] );
 		renderColumn( 'published',  snapshot.published   || [] );
 		renderColumn( 'failed',     snapshot.failed      || [] );
+		// Re-enable board Generate button once no active runs remain.
+		if ( ! snapshot.has_active_runs ) {
+			$( '#prab-board-generate-now' ).prop( 'disabled', false )
+				.text( ( cfg.strings && cfg.strings.newArticle ) || 'New Article' );
+		}
 	}
 
 	/**
@@ -225,22 +230,7 @@
 		} );
 	}
 
-	/** Initialise the board poller on DOM ready. */
-	$( function () {
-		$board = $( '#prab-board' );
-		if ( ! $board.length ) {
-			return;
-		}
 
-		// Inject the error banner after the heading.
-		$errorBanner = $( '<div class="prab-board-error" role="alert">' + escHtml( cfg.strings.pollError || '' ) + '</div>' );
-		$board.before( $errorBanner );
-
-		// Check if any generating card is present in the initial server render.
-		var hasActive = ( $board.find( '[data-column="generating"] .prab-board-card' ).length > 0 );
-		backoffFactor = hasActive ? 1 : 1; // Always start at base on first load.
-
-		schedulePoll( nextDelay( hasActive ) );
 	} );
 
 }( jQuery ) );