Move report queries onto Django's DB connection

[adobloug]

Scope and purpose

Fixes #4130.

The report subsystem read from the database through the legacy
nav.db.getConnection() cache (nav.ObjectCache), which is not thread-safe and
misbehaves under mod_wsgi's multithreaded workers — the same root cause as #4104.
This moves DatabaseResult, IPtree.get_subnets and MetaIP._createMetaMap
onto Django's django.db.connection, whose connections are thread-local, taking
reports off the fragile legacy cache.

metaIP dropped its legacy 'manage' database alias, which already resolved to
the same nav database (no db_manage in db.conf), so that is behaviourally a
no-op.

DB-API errors now surface as django.db.utils.* exceptions rather than
psycopg2.*. dbresult.py is updated to catch ProgrammingError / DataError
from django.db.utils; without this a malformed report SQL definition would
raise uncaught and produce an HTTP 500 — the regression flagged in the review of
#4111.

Split out from #4111 (issue #4104) as a separable change, per review feedback.

How to observe

The new integration tests exercise the real django.db.connection path,
including the error handling:

pytest tests/integration/report/dbresult_test.py \
       tests/integration/report/legacy_query_test.py

test_when_report_sql_is_malformed_then_error_is_set_without_raising and
test_when_report_input_type_is_invalid_then_data_error_is_set fail against the
old psycopg2 handlers (uncaught django.db.utils.* → HTTP 500) and pass with
the Django-wrapped handlers.

Contributor Checklist

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation — not applicable, internal change
• Linted/formatted the code with ruff
• Wrote the commit message so that the first line continues the sentence "If applied, this commit will ..."
• Based this pull request on the correct upstream branch (5.19.x, a bugfix affecting the latest stable version)
• If applicable: Created new issues if this PR does not fix the issue completely — not applicable, fully fixed
• If it's not obvious from a linked issue, described how to interact with NAV to observe the effects (see above)
• If this results in changes in the UI: Added screenshots — not applicable, no UI change
• If this adds a new Python source code file: Added the boilerplate header (both new test files have it)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud