[ADD] account_reconcile_bg: background processing for large bank reconciliations

[fw-bot-adhoc]

Implements background job processing for bank reconciliations when dealing with large payment batches to prevent timeouts and improve user experience.

Main features:

• Automatic background processing for large reconciliations (configurable threshold)
• User notifications when background reconciliation completes
• Seamless integration with existing bank reconciliation workflow
• No UI blocking: users can continue working while large batches process

Technical implementation:

• Override bank.rec.widget._js_action_validate() to detect large reconciliations
• Use base_bg to enqueue reconciliation jobs
• Add reconciliation_in_background flag to account.bank.statement.line
• Configurable via system parameter: account_reconcile_bg.lines_threshold

Includes comprehensive testing guide and unit tests.

Forward-Port-Of: #400

[roboadhoc]

[fw-bot-adhoc]

@feg-adhoc @rov-adhoc this PR targets 19.0 and is the last of the forward-port chain.

To merge the full chain, use

@roboadhoc r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[fw-bot-adhoc]

@feg-adhoc @rov-adhoc ci/runbot-modified-modules failed on this forward-port PR

[fw-bot-adhoc]

@feg-adhoc @rov-adhoc this forward port of #400 is awaiting action (not merged or closed).

[fw-bot-adhoc]

@feg-adhoc @rov-adhoc this forward port of #400 is awaiting action (not merged or closed).

[feg-adhoc]

@roboadhoc r+ nobump

[roboadhoc]

@feg-adhoc @rov-adhoc because this PR has multiple commits, I need to know how to merge it:

• merge to merge directly, using the PR as merge commit message
• rebase-merge to rebase and merge, using the PR as merge commit message
• rebase-ff to rebase and fast-forward

[feg-adhoc]

@roboadhoc r+ nobump