Add ruff linter/formatter for Python

- Add ruff as dev dependency
- Configure ruff in pyproject.toml with strict 88-char line limit
- Ignore B008 (FastAPI Depends pattern is standard)
- Allow longer lines in tests for readability
- Fix all lint issues in source files
- Add Makefile targets: lint-backend, format-backend, fix-backend

backend/invite_utils.py

@@ -1,4 +1,5 @@
 """Utilities for invite code generation and validation."""
+
 import random
 from pathlib import Path
 
@@ -13,11 +14,11 @@ assert len(BIP39_WORDS) == 2048, f"Expected 2048 BIP39 words, got {len(BIP39_WOR
 def generate_invite_identifier() -> str:
     """
     Generate a unique invite identifier.
-    
+
     Format: word1-word2-NN where:
     - word1, word2 are random BIP39 words
     - NN is a two-digit number (00-99)
-    
+
     Returns lowercase identifier.
     """
     word1 = random.choice(BIP39_WORDS)
@@ -29,7 +30,7 @@ def generate_invite_identifier() -> str:
 def normalize_identifier(identifier: str) -> str:
     """
     Normalize an invite identifier for comparison/lookup.
-    
+
     - Converts to lowercase
     - Strips whitespace
     """
@@ -39,22 +40,18 @@ def normalize_identifier(identifier: str) -> str:
 def is_valid_identifier_format(identifier: str) -> bool:
     """
     Check if an identifier has valid format (word-word-NN).
-    
+
     Does NOT check if words are valid BIP39 words.
     """
     parts = identifier.split("-")
     if len(parts) != 3:
         return False
-    
+
     word1, word2, number = parts
-    
+
     # Check words are non-empty
     if not word1 or not word2:
         return False
-    
-    # Check number is two digits
-    if len(number) != 2 or not number.isdigit():
-        return False
-    
-    return True
 
+    # Check number is two digits
+    return len(number) == 2 and number.isdigit()