Migrate from Gson and org.json to Jackson

Replaced Gson and org.json usage across the codebase with a unified Jackson-based implementation through a shared `Json` utility. Updated all JSON serialization and deserialization logic (`GsonJsonMapper`, HTTP client, task persistence, command handling, tests) to use Jackson's `ObjectMapper`. Removed the outdated dependencies from `pom.xml` and verified symmetry in payload serialization.

.junie/plans/consolidate-json-on-jackson.md

@@ -217,7 +217,7 @@ graph LR
 
 # Delivery Steps
 
-### * Step 1: Introduce Jackson and the shared Json utility
+### ✓ Step 1: Introduce Jackson and the shared Json utility
 Jackson is an explicit dependency and a single shared `ObjectMapper` is available to the codebase.
 
 - Add `com.fasterxml.jackson.core:jackson-databind` to `pom.xml`, pinned to the version already resolved via JDA (2.21.2), next to the existing `org.json`/`gson` entries (which stay for now so the build keeps compiling).
@@ -225,14 +225,14 @@ Jackson is an explicit dependency and a single shared `ObjectMapper` is availabl
 - Have `readObject`/`readArray` wrap Jackson's checked `JsonProcessingException` in an unchecked exception so existing call-site signatures stay unchanged.
 - Verify no shade-plugin relocation or filter rules in `pom.xml` reference `org.json` or `gson`.
 
-###   Step 2: Migrate the Gson call sites to Jackson
+### ✓ Step 2: Migrate the Gson call sites to Jackson
 Gson is no longer referenced anywhere in the source tree.
 
 - Delete `http/server/GsonJsonMapper.java` and remove its registration from the Javalin configuration, relying on Javalin 7's built-in Jackson `JsonMapper` instead.
 - Replace `new Gson().toJson(params)` at `http/WebModule.java:256` with `Json.write(params)`, reusing the shared mapper instead of allocating per request.
 - Confirm Javalin request/response serialization still behaves identically for the existing endpoints.
 
-###   Step 3: Migrate the HTTP client and its consumers
+### ✓ Step 3: Migrate the HTTP client and its consumers
 `WebModule` returns Jackson nodes and its callers read them.
 
 - Change `WebModule.getAsObject()` to return `ObjectNode` and `getAsArray()` to return `ArrayNode` (lines 133-143), parsing through `Json`.
@@ -240,7 +240,7 @@ Gson is no longer referenced anywhere in the source tree.
 - Update `command/crypto/CryptoPriceCommand.java` to read the crypto API response via `JsonNode` accessors.
 - Remove the now-unused `org.json` imports from these three files.
 
-###   Step 4: Migrate the task persistence layer
+### ✓ Step 4: Migrate the task persistence layer
 Tasks serialize and restore through Jackson while keeping the existing on-disk payload shape.
 
 - Change `PersistableTask.restore(JSONObject)` to `restore(ObjectNode)`.
@@ -249,7 +249,7 @@ Tasks serialize and restore through Jackson while keeping the existing on-disk p
 - Update `PersistableTaskRegistry` and `db/service/TaskService` to parse stored payload strings via `Json.readObject`.
 - Confirm `tasks/reaction/EmojiReactionRoleTask` still compiles unchanged against the new `EmojiTask` contract.
 
-###   Step 5: Update tests and drop the old dependencies
+### ✓ Step 5: Update tests and drop the old dependencies
 `org.json` and Gson are removed from `pom.xml` and the full build passes.
 
 - Rewrite payload construction in `PersistableTaskRegistryTest` and `EmojiReactionRoleTaskPersistenceTest` using `ObjectNode`.