[ADD] auto_invoice_email: Auto-send posted invoices from the configured days

- Modified the cron logic to generate and attach the invoice PDF when
sending email.
- Ensured the email includes the invoice PDF using QWeb report.
- Posted a message in the chatter with or without attachment, depending
on availability.
- Added fallback logic to handle missing attachments gracefully.

Author: rvar-odoo

auto_invoice_email/__init__.py

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+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+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">days</field>
+        <field name="active" eval="True" />
+    </record>
+</odoo>

auto_invoice_email/models/__init__.py

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+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+from . import res_config_settings
+from . import account_move

auto_invoice_email/models/account_move.py

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+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+from odoo import models, api
+from datetime import datetime, timedelta
+
+
+class AccountMove(models.Model):
+    _inherit = "account.move"
+
+    @api.model
+    def _auto_send_invoices(self):
+        # Using sudo to access system parameters that are not user-specific and may be restricted
+        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.with_context(force_send=True).send_mail(invoice.id, force_send=True)
+            invoice.message_post(body="Invoice automatically sent by cron job.", message_type="comment")

auto_invoice_email/models/res_config_settings.py

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+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+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>