Removes voided/flagged entry flags

Updates journal entry flags to align with Beancount's limited flag support.
Beancount only uses cleared (*) and pending (!) flags.

Removes the VOID and FLAGGED flags and recommends using tags instead
(e.g., "! + #voided" for voided entries, "! + #review" for flagged entries).

Updates the API to reflect this change, removing the ability to directly
"reject" an expense entry via the void flag.  Instead, instructs users to
add the #voided tag in Fava.

Updates reconciliation summary to count entries with voided/review tags
instead of voided/flagged flags.

models.py

@@ -15,11 +15,18 @@ class AccountType(str, Enum):
 
 
 class JournalEntryFlag(str, Enum):
-    """Transaction status flags (Beancount-style)"""
+    """Transaction status flags (Beancount-compatible)
+
+    Beancount only supports two user-facing flags:
+    - * (CLEARED): Completed transactions
+    - ! (PENDING): Transactions needing attention
+
+    For voided/flagged transactions, use tags instead:
+    - Voided: Use "!" flag + #voided tag
+    - Flagged: Use "!" flag + #review tag
+    """
     CLEARED = "*"      # Fully reconciled/confirmed
     PENDING = "!"      # Not yet confirmed/awaiting approval
-    FLAGGED = "#"      # Needs review/attention
-    VOID = "x"         # Voided/cancelled entry
 
 
 class Account(BaseModel):