Grow journal: time-stamped notes on gardens, beds and plantings (#52)

"Take notes through the season on each bed and plant." There is already
free-text notes on gardens, objects and plants, but it is a single mutable
field: writing "powdery mildew on the west bed" overwrites what you wrote in
June. The distinction worth keeping is that notes says what this thing IS, while
a journal entry says what HAPPENED, and when. Both stay.

garden_id is NOT NULL even when an entry is about a bed or a single plop, and
that is the whole trick: permission checks reuse requireGardenRole unchanged and
no second ACL path is invented. object_id/planting_id narrow the target; the
garden always anchors it. Because the garden anchors permission, the target is
checked to actually live in that garden — otherwise a valid object id from
someone else's garden would be storable here and then leak through the list
read. A plop-level entry that also names an object must name the right one, or
the two filters would disagree about what an entry is about.

observed_at is distinct from created_at: you write up Saturday's observations on
Sunday, and Saturday is the date that matters. Listing orders by observation,
newest first, with id breaking ties inside a day.

Roles follow the existing shape. Editors write, viewers read, strangers get
ErrNotFound. An author may edit their own entries; the garden owner may DELETE
any entry in their garden but may not edit one — rewriting somebody else's
observation under their name is a different act from removing it.

Deliberately not wired into the revision history, per the decision on #52:
entries are already append-shaped and individually versioned, and undoing a note
is just deleting it. There's a test asserting journal writes produce no change
sets, so that decision can't quietly reverse itself later.

Closes #52

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

internal/service/plants_test.go

@@ -184,12 +184,12 @@ func TestCreatePlantValidation(t *testing.T) {
 	owner := seedUser(t, s, "[email protected]")
 
 	bad := []PlantInput{
-		{Name: "", Category: domain.CategoryHerb, SpacingCM: 10, Color: "#4a7c3f", Icon: "🌿"},         // empty name
-		{Name: "x", Category: "spaceship", SpacingCM: 10, Color: "#4a7c3f", Icon: "🌿"},                // bad category
-		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 0, Color: "#4a7c3f", Icon: "🌿"},         // zero spacing
-		{Name: "x", Category: domain.CategoryHerb, SpacingCM: -5, Color: "#4a7c3f", Icon: "🌿"},        // negative spacing
-		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 10, Color: "nope", Icon: "🌿"},           // bad color
-		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 10, Color: "#4a7c3f", Icon: ""},          // empty icon
+		{Name: "", Category: domain.CategoryHerb, SpacingCM: 10, Color: "#4a7c3f", Icon: "🌿"},                                     // empty name
+		{Name: "x", Category: "spaceship", SpacingCM: 10, Color: "#4a7c3f", Icon: "🌿"},                                            // bad category
+		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 0, Color: "#4a7c3f", Icon: "🌿"},                                     // zero spacing
+		{Name: "x", Category: domain.CategoryHerb, SpacingCM: -5, Color: "#4a7c3f", Icon: "🌿"},                                    // negative spacing
+		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 10, Color: "nope", Icon: "🌿"},                                       // bad color
+		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 10, Color: "#4a7c3f", Icon: ""},                                     // empty icon
 		{Name: strings.Repeat("x", maxPlantNameLen+1), Category: domain.CategoryHerb, SpacingCM: 10, Color: "#4a7c3f", Icon: "🌿"}, // name too long
 		{Name: "x", Category: domain.CategoryHerb, SpacingCM: 10, Color: "#4a7c3f", Icon: "🌿", DaysToMaturity: ptrInt(0)},         // days must be >= 1
 	}