Add companion planting section to species detail page and redesign home page

Species detail now shows a "Companion Plants" card (left column, after
Ecology) with beneficial/antagonistic sub-lists. Each entry links to
the companion species and shows the mechanism.

Home page gains stats cards, quick-filter buttons, and a hero section.
Species list supports URL query params for quick-filter links.
New /api/v1/stats endpoint provides database counts.

i18n keys added for DE/EN: card.companion_plants, companions.beneficial_for,
companions.antagonistic_for, stat.*, filter.*, home.*.

herbapi-api/src/api/mod.rs

@@ -6,6 +6,7 @@ mod health;
 mod images;
 mod search;
 mod species;
+mod stats;
 mod suppliers;
 
 use axum::http::{header, HeaderValue, StatusCode};
@@ -44,6 +45,8 @@ pub fn router(state: AppState) -> Router {
         .route("/api/v1/images/{id}", delete(images::remove))
         // Image file serving (S3 proxy)
         .route("/img/{*path}", get(images::serve_image))
+        // Stats
+        .route("/api/v1/stats", get(stats::get_stats))
         // Search
         .route("/api/v1/search", get(search::search))
         // API docs

herbapi-api/src/api/stats.rs

@@ -0,0 +1,52 @@
+use axum::extract::State;
+use axum::Json;
+use serde::Serialize;
+
+use crate::error::Result;
+use crate::state::AppState;
+
+#[derive(Serialize)]
+pub struct DbStats {
+    pub families: i64,
+    pub species: i64,
+    pub cultivars: i64,
+    pub suppliers: i64,
+    pub companions: i64,
+    pub images: i64,
+}
+
+pub async fn get_stats(State(state): State<AppState>) -> Result<Json<DbStats>> {
+    let (families,): (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM families")
+            .fetch_one(&state.pool)
+            .await?;
+    let (species,): (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM species")
+            .fetch_one(&state.pool)
+            .await?;
+    let (cultivars,): (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM cultivars")
+            .fetch_one(&state.pool)
+            .await?;
+    let (suppliers,): (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM suppliers")
+            .fetch_one(&state.pool)
+            .await?;
+    let (companions,): (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM companion_relationships")
+            .fetch_one(&state.pool)
+            .await?;
+    let (images,): (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM images")
+            .fetch_one(&state.pool)
+            .await?;
+
+    Ok(Json(DbStats {
+        families,
+        species,
+        cultivars,
+        suppliers,
+        companions,
+        images,
+    }))
+}