docs: update README and tech docs for BadgerDB and thumbnails

- Reflect BadgerDB integration for metadata storage in features and
  architecture diagrams
- Add thumbnail generation and static asset subdirectories to project
  layout
- Document _MB configuration variants for size limits
- List main API request surfaces in docs/tech.md for better developer
  reference
- Align documentation with recent architectural and routing changes

README.md

@@ -1,19 +1,30 @@
 # WarpBox
 
 WarpBox is a small, self-hosted file sharing app with temporary upload boxes,
-simple download links, optional passwords, ZIP downloads, and a very deliberate
-retro desktop mood.
+simple download links, optional passwords, ZIP downloads, generated image
+thumbnails, and a very deliberate retro desktop mood.
 
 It is meant to feel quick: pick files, choose how long the box should live,
 upload, and share the link.
 
 ```mermaid
 flowchart LR
-    A[Choose files] --> B[Create box]
-    B --> C[Upload to box]
-    C --> D[Share link]
-    D --> E[Download files or ZIP]
-    E --> F[Expire or one-time cleanup]
+    User[Person in browser]
+    UI[WarpBox UI]
+    API[Go HTTP server]
+    Manifest[(Box manifest JSON)]
+    Files[(Uploaded files)]
+    Thumbs[(Thumbnail JPEGs)]
+    DB[(BadgerDB metadata)]
+
+    User --> UI
+    UI -->|create box / upload / poll status| API
+    API --> Manifest
+    API --> Files
+    API --> DB
+    Files -->|download files or build ZIP| API
+    Thumbs -->|preview URLs| UI
+    Files -->|scan image files| Thumbs
 ```
 
 ## Features
@@ -25,7 +36,8 @@ flowchart LR
 - One-time download mode for ZIP-only handoff.
 - Background thumbnails for image files.
 - Plain filesystem storage, with JSON manifests next to uploaded files.
-- No database required.
+- Local BadgerDB metadata store for users, tags, sessions, and settings.
+- No external database service required.
 
 ## How It Fits Together
 
@@ -36,11 +48,13 @@ flowchart TB
     Manifest[Box manifest JSON]
     Files[Uploaded files]
     Thumbs[Generated thumbnails]
+    DB[(BadgerDB metadata)]
 
-    Browser -->|create box / upload / poll| Server
+    Browser -->|POST /box, uploads, status polls| Server
     Server --> Manifest
     Server --> Files
     Server --> Thumbs
+    Server --> DB
     Thumbs -->|preview URLs| Browser
     Files -->|downloads / ZIP| Browser
 ```
@@ -101,6 +115,8 @@ settings remain environment controlled.
 | `WARPBOX_THUMBNAIL_BATCH_SIZE` | `10` | Number of pending thumbnails processed per worker pass. |
 | `WARPBOX_THUMBNAIL_INTERVAL_SECONDS` | `30` | Delay between thumbnail worker passes. |
 
+Size limits also accept `_MB` variants for the same settings.
+
 Example:
 
 ```bash
@@ -144,23 +160,29 @@ data/db/
 ## Project Layout
 
 ```text
-cmd/            CLI entrypoint
-lib/server/     HTTP handlers and server setup
-lib/routing/    Route registration
-lib/boxstore/   Box storage, manifests, downloads, thumbnails
-lib/config/     Typed environment and runtime settings config
-lib/metastore/  BadgerDB metadata store for users, tags, settings, sessions
-lib/helpers/    Small shared helpers
-lib/models/     Shared request/response models
-templates/      Server-rendered HTML
-static/         CSS, JavaScript, fonts, icons, and images
-docs/           Project documentation
+cmd/                CLI entrypoint
+lib/server/         HTTP handlers and server setup
+lib/routing/        Route registration
+lib/boxstore/       Box storage, manifests, downloads, thumbnails
+lib/config/         Typed environment and runtime settings config
+lib/metastore/      BadgerDB metadata store for users, tags, settings, sessions
+lib/helpers/        Small shared helpers
+lib/models/         Shared request/response models
+templates/          Server-rendered HTML
+static/css/         Stylesheets
+static/js/          Browser scripts
+static/img/         Icons, sprites, and backgrounds
+static/fonts/       Bitmap/pixel fonts
+static/cursors/     Custom cursor packs
+static/popups/      HTML popup content
+docs/               Project documentation
 ```
 
 ## Notes
 
-WarpBox is intentionally simple. It uses the local filesystem as its backing
-store, relies on generated box IDs for share links, and keeps most behavior
-easy to follow from the Go handlers and the small browser scripts.
+WarpBox is intentionally simple. It uses the local filesystem for box data,
+BadgerDB for app metadata, relies on generated box IDs for share links, and
+keeps most behavior easy to follow from the Go handlers and the small browser
+scripts.
 
 For a short implementation overview, see [docs/tech.md](docs/tech.md).