fix!: handle multiple companies' bank accounts with one EBICS User

[PatrickDEissler]

Solves: #404

Further note:
process_camt_document had to be adjusted, because it relied on transactions for one Company per EBICS User.

[greptile-apps]

Confidence Score: 4/5

This PR is not safe to merge until ambiguous historical IBAN matches stop cross-company routing and existing duplicate records remain administratively manageable.

The importer still continues with the first account when historical duplicates match a statement, while the newly added global validation does not migrate those records and instead prevents either duplicate account from being saved during routine maintenance.

Files Needing Attention: banking/ebics/utils.py and banking/overrides/bank_account.py

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[EBICS User downloads CAMT files] --> B[Read statement IBAN]
  B --> C[Find enabled company Bank Accounts by bank and IBAN]
  C --> D{Number of matches}
  D -->|None| E[Log error and skip statement]
  D -->|One| F[Create transactions for selected account]
  D -->|Multiple historical matches| G[Log ambiguity]
  G --> H[Select first returned account]
  H --> F

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Prompt To Fix All With AI

### Issue 1
banking/overrides/bank_account.py:41-46
**Historical duplicates block account updates**

When an existing Bank Account belongs to a historical duplicate-IBAN pair, this unscoped validation finds the other record and rejects every save, causing routine updates such as assigning a fee account or changing account metadata to fail until administrators manually resolve the historical data.

---

For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (2): Last reviewed commit: "fix(Bank Account): avoid duplicate IBANs..." | Re-trigger Greptile}

[barredterra]

I think having multiple Bank Accounts with the same IBAN is a configuration error, not a valid scenario. We can validate and log errors, but should not handle it in sync.

IMO the accurate fix should be:

• replace company field in Ebics User with a data field organization_name (and add a patch) - this is still needed for certificates
• drop company from get_bank_account() (param and argument)
• drop company from process_camt_document() and all downstream function calls. No need to fetch it explicitly, it should be set via Bank Transaction's fetch_from.
• add duplicate IBAN validation to Bank Account
• in get_bank_account, use get_all, log an error if more than 1 result, return the first result

[PatrickDEissler]

I think having multiple Bank Accounts with the same IBAN is a configuration error, not a valid scenario. We can validate and log errors, but should not handle it in sync.

IMO the accurate fix should be:

• replace company field in Ebics User with a data field organization_name (and add a patch) - this is still needed for certificates
• drop company from get_bank_account() (param and argument)
• drop company from process_camt_document() and all downstream function calls. No need to fetch it explicitly, it should be set via Bank Transaction's fetch_from.
• add duplicate IBAN validation to Bank Account
• in get_bank_account, use get_all, log an error if more than 1 result, return the first result

@barredterra
Thanks! I implemented all your suggestions.