fix: remove unsupported issues section from golangci-lint v2.x configs

The golangci-lint v2.8.0 schema validation rejected all properties
in the issues section:
- exclude-use-default
- exclude-dirs
- exclude-files
- exclude
- max-issues-per-linter
- max-same-issues

Solution: Removed the entire issues section from both config files.
Linter behavior is now controlled exclusively through linters.settings,
which is properly configured for govet, errcheck, gosec, gocritic, etc.

Changes to backend/.golangci-fast.yml and backend/.golangci.yml:
- Removed issues section entirely (v2.x schema incompatible)
- Retained all linter-specific settings under linters.settings
- Linters will run with their configured settings and default behaviors

This resolves the jsonschema validation error:
"additional properties ... not allowed"

Fixes: #666 (golangci-lint v2.x schema validation)

backend/.golangci-fast.yml

@@ -31,13 +31,3 @@ linters:
         - G501  # Blacklisted import crypto/md5
         - G502  # Blacklisted import crypto/des
         - G503  # Blacklisted import crypto/rc4
-
-issues:
-  exclude-use-default: false
-  exclude-dirs:
-    - vendor
-  exclude-files:
-    - ".*\\.gen\\.go$"
-  exclude:
-    # Allow test-specific patterns
-    - 'Error return value of `.*(json\.Unmarshal|SetPassword|CreateProvider).*` is not checked'

backend/.golangci.yml

@@ -55,18 +55,3 @@ linters:
         - (*database/sql.Rows).Close
         - (gorm.io/gorm.Migrator).DropTable
         - (*net/http.Response.Body).Close
-
-issues:
-  exclude-use-default: false
-  exclude:
-    # Gosec exclusions - be specific to avoid hiding real issues
-    # G104: Ignoring return values - already checked by errcheck
-    - "G104:"
-    # Allow shadow warnings in specific patterns
-    - "shadows declaration"
-  exclude-dirs:
-    - vendor
-  exclude-files:
-    - ".*_test\\.go$"  # Test-specific exclusions handled via patterns
-  max-issues-per-linter: 0
-  max-same-issues: 0