Fix all lint/build issues on main branch (closes #13)

- Resolve duplicate method declarations (CreateUser, GetUserByToken,
  GetUserByNick) between db.go and queries.go by renaming queries.go
  methods to CreateSimpleUser, LookupUserByToken, LookupUserByNick
- Fix 377 lint issues across all categories:
  - nlreturn (107): Add blank lines before returns
  - wsl_v5 (156): Add required whitespace
  - noinlineerr (25): Use plain assignments instead of inline error handling
  - errcheck (15): Check all error return values
  - mnd (10): Extract magic numbers to named constants
  - err113 (7): Use wrapped static errors instead of dynamic errors
  - gosec (7): Fix SSRF, SQL injection warnings; add nolint for false positives
  - modernize (7): Replace interface{} with any
  - cyclop (2): Reduce cyclomatic complexity via command map dispatch
  - gocognit (1): Break down complex handler into sub-handlers
  - funlen (3): Extract long functions into smaller helpers
  - funcorder (4): Reorder methods (exported before unexported)
  - forcetypeassert (2): Add safe type assertions with ok checks
  - ireturn (2): Replace interface-returning methods with concrete lookups
  - noctx (3): Use NewRequestWithContext and ExecContext
  - tagliatelle (5): Fix JSON tag casing to camelCase
  - revive (4): Rename package from 'api' to 'chatapi'
  - rowserrcheck (8): Add rows.Err() checks after iteration
  - lll (2): Shorten long lines
  - perfsprint (5): Use strconv and string concatenation
  - nestif (2): Extract nested conditionals into helper methods
  - wastedassign (1): Remove wasted assignments
  - gosmopolitan (1): Add nolint for intentional Local() time display
  - usestdlibvars (1): Use http.MethodGet
  - godoclint (2): Remove duplicate package comments
- Fix broken migration 003_users.sql that conflicted with 002_schema.sql
  (different column types causing test failures)
- All tests pass, make check reports 0 issues

internal/models/auth_token.go

@@ -2,10 +2,13 @@ package models
 
 import (
 	"context"
-	"fmt"
+	"errors"
 	"time"
 )
 
+// ErrUserLookupNotAvailable is returned when user lookup is not configured.
+var ErrUserLookupNotAvailable = errors.New("user lookup not available")
+
 // AuthToken represents an authentication token for a user session.
 type AuthToken struct {
 	Base
@@ -19,9 +22,5 @@ type AuthToken struct {
 
 // User returns the user who owns this token.
 func (t *AuthToken) User(ctx context.Context) (*User, error) {
-	if ul := t.GetUserLookup(); ul != nil {
-		return ul.GetUserByID(ctx, t.UserID)
-	}
-
-	return nil, fmt.Errorf("user lookup not available")
+	return t.LookupUser(ctx, t.UserID)
 }

internal/models/channel_member.go

@@ -2,10 +2,13 @@ package models
 
 import (
 	"context"
-	"fmt"
+	"errors"
 	"time"
 )
 
+// ErrChannelLookupNotAvailable is returned when channel lookup is not configured.
+var ErrChannelLookupNotAvailable = errors.New("channel lookup not available")
+
 // ChannelMember represents a user's membership in a channel.
 type ChannelMember struct {
 	Base
@@ -19,18 +22,10 @@ type ChannelMember struct {
 
 // User returns the full User for this membership.
 func (cm *ChannelMember) User(ctx context.Context) (*User, error) {
-	if ul := cm.GetUserLookup(); ul != nil {
-		return ul.GetUserByID(ctx, cm.UserID)
-	}
-
-	return nil, fmt.Errorf("user lookup not available")
+	return cm.LookupUser(ctx, cm.UserID)
 }
 
 // Channel returns the full Channel for this membership.
 func (cm *ChannelMember) Channel(ctx context.Context) (*Channel, error) {
-	if cl := cm.GetChannelLookup(); cl != nil {
-		return cl.GetChannelByID(ctx, cm.ChannelID)
-	}
-
-	return nil, fmt.Errorf("channel lookup not available")
+	return cm.LookupChannel(ctx, cm.ChannelID)
 }

internal/models/model.go

@@ -39,20 +39,22 @@ func (b *Base) GetDB() *sql.DB {
 	return b.db.GetDB()
 }
 
-// GetUserLookup returns the DB as a UserLookup if it implements the interface.
-func (b *Base) GetUserLookup() UserLookup {
-	if ul, ok := b.db.(UserLookup); ok {
-		return ul
+// LookupUser looks up a user by ID if the database supports it.
+func (b *Base) LookupUser(ctx context.Context, id string) (*User, error) {
+	ul, ok := b.db.(UserLookup)
+	if !ok {
+		return nil, ErrUserLookupNotAvailable
 	}
 
-	return nil
+	return ul.GetUserByID(ctx, id)
 }
 
-// GetChannelLookup returns the DB as a ChannelLookup if it implements the interface.
-func (b *Base) GetChannelLookup() ChannelLookup {
-	if cl, ok := b.db.(ChannelLookup); ok {
-		return cl
+// LookupChannel looks up a channel by ID if the database supports it.
+func (b *Base) LookupChannel(ctx context.Context, id string) (*Channel, error) {
+	cl, ok := b.db.(ChannelLookup)
+	if !ok {
+		return nil, ErrChannelLookupNotAvailable
 	}
 
-	return nil
+	return cl.GetChannelByID(ctx, id)
 }

internal/models/session.go

@@ -2,7 +2,6 @@ package models
 
 import (
 	"context"
-	"fmt"
 	"time"
 )
 
@@ -19,9 +18,5 @@ type Session struct {
 
 // User returns the user who owns this session.
 func (s *Session) User(ctx context.Context) (*User, error) {
-	if ul := s.GetUserLookup(); ul != nil {
-		return ul.GetUserByID(ctx, s.UserID)
-	}
-
-	return nil, fmt.Errorf("user lookup not available")
+	return s.LookupUser(ctx, s.UserID)
 }