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/tests/test_validation.py

@@ -1,12 +1,11 @@
 """Tests for profile field validation."""
-import pytest
 
 from validation import (
     validate_contact_email,
-    validate_telegram,
-    validate_signal,
     validate_nostr_npub,
     validate_profile_fields,
+    validate_signal,
+    validate_telegram,
 )
 
 
@@ -140,13 +139,17 @@ class TestValidateNostrNpub:
         assert validate_nostr_npub(self.VALID_NPUB) is None
 
     def test_wrong_prefix(self):
-        result = validate_nostr_npub("nsec1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqwcv5dz")
+        result = validate_nostr_npub(
+            "nsec1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqwcv5dz"
+        )
         assert result is not None
         assert "npub" in result.lower()
 
     def test_invalid_checksum(self):
         # Change last character to break checksum
-        result = validate_nostr_npub("npub1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqsutgpd")
+        result = validate_nostr_npub(
+            "npub1qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqsutgpd"
+        )
         assert result is not None
         assert "checksum" in result.lower()
 
@@ -155,7 +158,9 @@ class TestValidateNostrNpub:
         assert result is not None
 
     def test_not_starting_with_npub1(self):
-        result = validate_nostr_npub("npub2qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqsutgpc")
+        result = validate_nostr_npub(
+            "npub2qqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqsutgpc"
+        )
         assert result is not None
         assert "npub1" in result
 
@@ -206,4 +211,3 @@ class TestValidateProfileFields:
             nostr_npub="",
         )
         assert errors == {}
-