Address Gadfly review on #7: garden cap, dim validation, shared errors

Fixes from the PR #26 adversarial review (graded 18 real / 0 false positive).

Correctness / security
- maxGardenCM fixed to 10_000 (100 m), matching its comment — it was
  100_000 cm (1 km), 10x too lax (5 models flagged this).
- Dimension validation now rejects NaN/Inf (which slip past naive
  comparisons) and subnormal-tiny positives, via a finite [1cm, 100m]
  check. Name (200) and notes (10_000) are length-capped so untrusted
  input can't balloon storage.
- Update version binding is `required,min=1`, so a negative/zero version
  is a 400, not a 409.

Maintainability / performance
- One unified writeServiceError (new errors.go) maps every auth + resource
  sentinel; writeResourceError removed. writeVersionConflict and
  parseIDParam moved to errors.go (shared, not in the gardens feature file).
- Request structs share an embedded gardenFields (one toInput).
- CreateGarden uses INSERT ... RETURNING (one round-trip).
- ListGardensForOwner has a defensive LIMIT (pagination is post-v1).

Tests: name/notes length, NaN/Inf/subnormal dims, dimension-at-cap valid,
negative/zero version -> 400.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

internal/service/gardens.go

@@ -2,17 +2,22 @@ package service
 
 import (
 	"context"
+	"math"
 	"strings"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
 )
 
-// Garden sizing bounds. A new garden with no dimensions defaults to a 10 m
-// square; the max is a generous sanity cap (100 m) that also guards against
-// absurd or overflow-y values.
+// Garden sizing and field bounds. A new garden with no dimensions defaults to a
+// 10 m square; dimensions must be finite and within [1 cm, 100 m] — a generous
+// sanity range that also rejects NaN/Inf and absurd values. Name/notes are
+// length-capped so untrusted input can't balloon storage.
 const (
-	defaultGardenCM = 1000
-	maxGardenCM     = 100_000
+	defaultGardenCM   = 1000   // 10 m
+	minGardenCM       = 1      // 1 cm
+	maxGardenCM       = 10_000 // 100 m
+	maxGardenNameLen  = 200
+	maxGardenNotesLen = 10_000
 )
 
 // gardenRole ranks a user's access to a garden. Higher includes lower
@@ -118,12 +123,17 @@ func (s *Service) DeleteGarden(ctx context.Context, actorID, gardenID int64) err
 // filled in (create); without it, every field must be supplied (update).
 func gardenFromInput(in GardenInput, applyDefaults bool) (*domain.Garden, error) {
 	name := strings.TrimSpace(in.Name)
-	if name == "" {
+	if name == "" || len(name) > maxGardenNameLen {
+		return nil, domain.ErrInvalidInput
+	}
+	notes := strings.TrimSpace(in.Notes)
+	if len(notes) > maxGardenNotesLen {
 		return nil, domain.ErrInvalidInput
 	}
 
-	// 0 means "unset": defaulted on create, rejected on update. A negative value
-	// is always an explicit error (never silently corrected to the default).
+	// 0 means "unset": defaulted on create, rejected on update. Anything else
+	// must be a finite length in range — this also rejects negatives, NaN, Inf,
+	// and subnormal-tiny values (which are > 0 but below 1 cm).
 	width, height := in.WidthCM, in.HeightCM
 	if applyDefaults {
 		if width == 0 {
@@ -133,7 +143,7 @@ func gardenFromInput(in GardenInput, applyDefaults bool) (*domain.Garden, error)
 			height = defaultGardenCM
 		}
 	}
-	if width <= 0 || height <= 0 || width > maxGardenCM || height > maxGardenCM {
+	if !validDimensionCM(width) || !validDimensionCM(height) {
 		return nil, domain.ErrInvalidInput
 	}
 
@@ -150,6 +160,13 @@ func gardenFromInput(in GardenInput, applyDefaults bool) (*domain.Garden, error)
 		WidthCM:  width,
 		HeightCM: height,
 		UnitPref: unit,
-		Notes:    strings.TrimSpace(in.Notes),
+		Notes:    notes,
 	}, nil
 }
+
+// validDimensionCM reports whether v is a finite length within the allowed
+// garden range. Guards against NaN/Inf (which slip past naive < / > comparisons)
+// and subnormal-tiny positives.
+func validDimensionCM(v float64) bool {
+	return !math.IsNaN(v) && !math.IsInf(v, 0) && v >= minGardenCM && v <= maxGardenCM
+}