ORM Bugs

[JohananOppongAmoateng]

This PR addresses multiple issues to improve django-polymorphic's reliability and usability.
Q-objects couldn't accept string model references, causing circular dependency issues in Meta constraints when a base model needed to reference a derived model defined later.

Saving polymorphic objects from one database to another caused ForeignKeyViolation because polymorphic_ctype_id referenced ContentType IDs from the source database.

Database routers weren't respected - ContentType was always fetched from default database even when routers specified a different database.

Closes #486
Closes #505
Closes #446

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.88%. Comparing base (9f5e0fc) to head (c47046a).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/polymorphic/query_translate.py	96.55%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #757      +/-   ##
==========================================
+ Coverage   78.28%   78.88%   +0.59%     
==========================================
  Files          26       26              
  Lines        1543     1591      +48     
  Branches      244      254      +10     
==========================================
+ Hits         1208     1255      +47     
- Misses        260      261       +1     
  Partials       75       75              

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