fix: Add mypy easy fixes to repository.

[bvanelli]

Summary

This PR adds initial mypy type checking support and fixes the easy/straightforward type errors across the codebase. It reduces the total mypy error count from 511 errors across 14 files down to 232 errors across 3 files.

Error reduction: 279 errors fixed (54.6% reduction)

Supports #190

Mypy report comparison

Per-file error count

File	main	This PR	Delta
actual/rules.py	218	214	-4
actual/queries.py	140	15	-125
actual/__init__.py	57	3	-54
actual/database.py	52	0	-52
actual/cli/main.py	19	0	-19
actual/budgets.py	7	0	-7
actual/schedules.py	4	0	-4
actual/api/models.py	3	0	-3
actual/protobuf_models.py	3	0	-3
actual/utils/changeset.py	2	0	-2
actual/api/__init__.py	2	0	-2
actual/cli/config.py	2	0	-2
actual/api/bank_sync.py	1	0	-1
actual/exceptions.py	1	0	-1
Total	511	232	-279

Error categories

Error type	main	This PR	Delta
[operator]	128	156	+28
[arg-type]	91	32	-59
[union-attr]	87	14	-73
[str]	71	77	+6
[assignment]	51	6	-45
[bool]	38	0	-38
[call-overload]	10	10	0
[attr-defined]	7	5	-2
[return-value]	6	1	-5
[index]	4	4	0
[import-untyped]	3	0	-3
[call-arg]	3	0	-3
[var-annotated]	3	0	-3
[misc]	2	3	+1
[return]	2	0	-2

Note: The [operator] and [str] increases come from actual/rules.py, where stricter typing now surfaces previously hidden issues with enum comparisons. These are not regressions — they are pre-existing issues now made visible.

Remaining errors (232)

The remaining 232 errors are concentrated in 3 files:

• actual/rules.py (214) — Mostly [operator] and [str] errors from enum/literal type comparisons in the rules engine. These require deeper refactoring of the rules type system.
• actual/queries.py (15) — Primarily [call-overload] errors from SQLModel's Session.exec() overload resolution, which is a known upstream issue.
• actual/__init__.py (3) — Minor type narrowing issues.

Changes made

• Added mypy configuration to pyproject.toml with types-python-dateutil stub dependency
• Fixed type annotations across 15 source files and 4 test files
• Added proper Optional types, type narrowing (assert, if guards), and return type annotations
• Fixed None-safety issues (union-attr), argument type mismatches (arg-type), and assignment incompatibilities
• Fixed missing return statements and import stubs

🤖 This report has been generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.45238% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.60%. Comparing base (6410640) to head (5e85357).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
actual/__init__.py	90.81%	9 Missing ⚠️
actual/queries.py	92.30%	6 Missing ⚠️
actual/database.py	93.90%	5 Missing ⚠️
actual/exceptions.py	0.00%	1 Missing ⚠️
actual/rules.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
- Coverage   98.01%   97.60%   -0.42%     
==========================================
  Files          21       21              
  Lines        3131     3212      +81     
==========================================
+ Hits         3069     3135      +66     
- Misses         62       77      +15     

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