Refactor domain_utils.py overlap helpers, add ONE_DAY constant, remove inflated comments, and standardize _add_row() calls

AGENTS.md

@@ -71,6 +71,32 @@ Two sources with shared patterns:
 - **comdirect**: `ComdirectGiro`, `ComdirectCreditCard`, `ComdirectDepot`, import to `comdirect_giro` table
 - **unified**: `Transaction`, `TransactionDb` – both ingest pipelines project their staging rows (`import_b4w`, `comdirect_giro`) into one `transactions` table via `src/ingest/transactions.py` (`map_*` mappers + `consolidate_transactions`). Dedup uses a business-key `hash_id` (source + IBAN + value date + amount + text + counterparty) with a UNIQUE constraint and SQLite `INSERT OR IGNORE`, so re-imports are idempotent. Amounts are canonical signed (negative = money out). Depot/credit-card/tagesgeld are not projected yet.
 
+## Category Budget Management (NEW)
+
+Budget tracking for categories with time-bounded or unbounded periods:
+
+```python
+from src.models.category import Category, CategoryBudget
+from datetime import date
+
+cat = Category(name="Lebenshaltung")
+budget = CategoryBudget(
+    category=cat,
+    amount_eur=150.00,
+    valid_from=date(2026, 10, 1),   # Optional: None for unbounded start
+    valid_to=date(2026, 11, 30),     # Optional: None for unbounded end
+)
+
+# Set/replace budget - handles overlap automatically:
+from src.domain_utils import set_category_budget
+set_category_budget(db_session, budget)
+```
+
+The `set_category_budget()` function intelligently handles three scenarios:
+1. **No overlap**: Adds new budget alongside existing ones
+2. **Partial overlap**: Splits existing budget into non-overlapping parts + adds new budget  
+3. **Full overlap**: Replaces existing budget entirely
+
 All models inherit from common base in `src/models/common.py`:
 - `_hash()`: SHA256-based deduplication key
 - `_parse_date_german()`: Handles "07.09.2026" format and values like "offen", "--"