feat(cost): per-run cost governor — reserve-before-call enforcement

Pipeline v2 Phase 1, commit 3/4. Cost_Governor reserves each call's
worst-case estimate (prompt chars/4 + max_tokens via the #18 pricing
chain) against the run ledger row with one atomic conditional UPDATE
(the #10 lock discipline) BEFORE dispatch, then settles to actuals after
each response; failed calls release their hold. Integrated inside
OpenRouter_Provider::send_chat_completion so EVERY text-LLM call path is
guarded with zero call-site changes; the image curl_multi batch reserves
its summed estimate in Image_Pipeline before generate_image_batch.

Breach: run halted (sticky), overage recorded, Cost_Ceiling_Exception
aborts the un-dispatched call, process_next_queued_article aborts the
remaining queued articles of halted/failed runs, and the Review Queue
shows a 'Halted runs' notice (runs halted pre-post have no draft to
list). Ceiling = prautoblogger_per_run_cost_ceiling_usd setting (default
constant $0.50, snapshotted at run start, 0 = disabled). Standalone
no-run calls, the monthly cap, and the AI Gateway path are unchanged.

Under-300: provider API-key fetch+validation moved verbatim to
OpenRouter_Request_Builder::resolve_api_key().

Agent-Session: pipeline-v2-phase1-engineer-20260612

includes/admin/class-settings-fields-extended.php

@@ -113,6 +113,16 @@ class PRAutoBlogger_Settings_Fields_Extended {
 				'step'        => '0.01',
 				'description' => __( 'Hard stop when reached. 0 = unlimited.', 'prautoblogger' ),
 			),
+			array(
+				'id'          => 'prautoblogger_per_run_cost_ceiling_usd',
+				'label'       => __( 'Per-Run Cost Ceiling (USD)', 'prautoblogger' ),
+				'type'        => 'number',
+				'section'     => 'prautoblogger_schedule',
+				'default'     => PRAUTOBLOGGER_DEFAULT_RUN_CEILING_USD,
+				'min'         => 0,
+				'step'        => '0.01',
+				'description' => __( 'Hard per-run stop: every API call reserves its worst-case cost against this ceiling before dispatch; a breach halts the run and routes it to the Review Queue. Snapshotted at run start. 0 = disabled.', 'prautoblogger' ),
+			),
 
 			// ── Publishing ──────────────────────────────────────────────
 			array(