feat(content): production content-pack loader + referenced-definition preflight

Production real-profile content is loaded via an explicit path, NOT the dev-only
OPENFUT_DEV_CONTENT_GAMES gate:
- Config.content_packs from env OPENFUT_CONTENT_PACKS (comma-sep file paths).
- CardDb::load_pack(path): merge an explicit CardDefinition[] production pack.
- app::build loads dev packs then production packs.

Preflight (app::build, always on): every owned_cards.card_id MUST resolve to a
loaded CardDefinition. A real profile with owned players but even ONE missing
definition fails LOUDLY instead of silently serving an empty /collection; an
empty owned_cards table (fresh DB / tests) passes.

2 preflight integration tests (missing def fails, loaded def passes). clippy
-D warnings clean; full suite 151 tests green.

src/config.rs

@@ -1,4 +1,5 @@
 use anyhow::Result;
+use std::path::PathBuf;
 
 #[derive(Debug, Clone)]
 pub struct Config {
@@ -10,6 +11,11 @@ pub struct Config {
     /// loaded IN ADDITION to the default `data/cards` catalog. Empty by default —
     /// default/test content is never affected unless a game is named here.
     pub dev_content_games: Vec<String>,
+    /// Explicit PRODUCTION content pack file paths (each a `CardDefinition[]`
+    /// JSON), loaded IN ADDITION to `data/cards` and any dev pack. This is the
+    /// production real-profile content path — deliberately NOT gated behind the
+    /// dev-only `dev_content_games`.
+    pub content_packs: Vec<PathBuf>,
 }
 
 impl Config {
@@ -33,6 +39,16 @@ impl Config {
                         .collect()
                 })
                 .unwrap_or_default(),
+            content_packs: std::env::var("OPENFUT_CONTENT_PACKS")
+                .ok()
+                .map(|v| {
+                    v.split(',')
+                        .map(str::trim)
+                        .filter(|s| !s.is_empty())
+                        .map(PathBuf::from)
+                        .collect()
+                })
+                .unwrap_or_default(),
         })
     }
 }