[ADD] account: Auto-send posted invoices older than configured days

The motivation behind this PR:
- To automate the process of sending posted invoices via email for all invoices
 older than a configurable number of days. This ensures timely communication
 with customers without requiring manual intervention.

The behavior before this PR:
- Only invoices with an exact `invoice_date` equal to today's date minus the
 configured number of days (`= target_date`) were selected. Older invoices were
 skipped, even if they were still unsent.

The desired behavior after this PR:
- Now, the system selects all posted invoices where `invoice_date` is less than
 the calculated `target_date`, allowing pending older invoices to be sent
 automatically via the scheduled cron job.

Author: rvar-odoo

auto_invoice_email/__init__.py

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+from . import models

auto_invoice_email/__manifest__.py

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+{
+    "name": "Auto Invoice Email",
+    "version": "1.0",
+    "summary": "Automatically send posted invoices by email after configured days",
+    "category": "Accounting",
+    "author": "Rohit",
+    "depends": ["account"],
+    "data": [
+        "views/res_config_settings_views.xml",
+        "data/auto_send_cron.xml",
+    ],
+    "installable": True,
+    "license": "LGPL-3",
+}

auto_invoice_email/data/auto_send_cron.xml

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+<odoo>
+    <record id="ir_cron_auto_send_invoices" model="ir.cron">
+        <field name="name">Auto Send Posted Invoices</field>
+        <field name="model_id" ref="account.model_account_move"/>
+        <field name="user_id" ref="base.user_root" />
+        <field name="state">code</field>
+        <field name="code">model._auto_send_invoices</field>
+        <field name="interval_number">1</field>
+        <field name="interval_type">minutes</field>
+        <field name="active" eval="True" />
+    </record>
+</odoo>

auto_invoice_email/models/__init__.py

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+from . import res_config_settings
+from . import account_move

auto_invoice_email/models/account_move.py

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+from odoo import models, api
+from datetime import datetime, timedelta
+
+
+class AccountMove(models.Model):
+    _inherit = "account.move"
+
+    @api.model
+    def _auto_send_invoices(self):
+        days = int(
+            self.env["ir.config_parameter"]
+            .sudo()
+            .get_param("auto_invoice_email.days", default=0)
+        )
+        if days <= 0:
+            return
+
+        target_date = datetime.today().date() - timedelta(days=days)
+        invoices = self.search(
+            [
+                ("state", "=", "posted"),
+                ("invoice_date", "=", target_date),
+                ("move_type", "in", ("out_invoice", "out_refund")),
+            ]
+        )
+        template = self.env.ref("account.email_template_edi_invoice")
+        for invoice in invoices:
+            template.send_mail(invoice.id, force_send=True)
+            attachment = self.env["ir.attachment"].search(
+                [
+                    ("res_model", "=", "account.move"),
+                    ("res_id", "=", invoice.id),
+                    ("mimetype", "=", "application/pdf"),
+                ],
+                limit=1,
+            )
+            subtype = self.env.ref("mail.mt_note")
+
+            body = "Invoice automatically sent by cron job."
+            if attachment:
+                invoice.sudo().message_post(
+                    body=body,
+                    attachment_ids=[attachment.id],
+                    message_type="comment",
+                    subtype_id=subtype.id,
+                )
+            else:
+                invoice.sudo().message_post(
+                    body=body, message_type="comment", subtype_id=subtype.id
+                )

auto_invoice_email/models/res_config_settings.py

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+from odoo import models, fields, api
+
+
+class ResConfigSettings(models.TransientModel):
+    _inherit = "res.config.settings"
+
+    auto_send_invoice_days = fields.Integer(string="Send invoices after (days)")
+
+    def set_values(self):
+        super().set_values()
+        self.env["ir.config_parameter"].sudo().set_param(
+            "auto_invoice_email.days", self.auto_send_invoice_days
+        )
+
+    @api.model
+    def get_values(self):
+        res = super().get_values()
+        res.update(
+            {
+                "auto_send_invoice_days": int(
+                    self.env["ir.config_parameter"]
+                    .sudo()
+                    .get_param("auto_invoice_email.days", default=0)
+                )
+            }
+        )
+        return res

auto_invoice_email/views/res_config_settings_views.xml

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+<odoo>
+    <record id="view_res_config_settings_inherit_auto_invoice" model="ir.ui.view">
+        <field name="name">res.config.settings.inherit.auto.invoice</field>
+        <field name="model">res.config.settings</field>
+        <field name="inherit_id" ref="account.res_config_settings_view_form"/>
+        <field name="arch" type="xml">
+            <xpath expr="//block[@id='invoicing_settings']" position="inside">
+                <setting id="auto_send_invoice_setting" help="Send invoices automatically after X days">
+                    <field name="auto_send_invoice_days" placeholder="Enter days"/>
+                </setting>
+            </xpath>
+        </field>
+    </record>
+</odoo>