Automatically run jacket processing

This commit makes it so jacket processing is automatically run if any of
it's outputs are missing from the filesystem, or if the hash of the raw
jacket directory has changes since the last recorded value.

Moreover, most assets and all fonts are now embedded in the binary!

migrations/05-metadata/up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS metadata (
+  -- We only a single metadata row in the entire db
+  id INTEGER PRIMARY KEY CHECK (id == 0),
+
+  -- The last hash computed for the directory
+  -- containing all the raw jackets. If this
+  -- hash changes, every jacket is reprocessed.
+  raw_jackets_hash TEXT NOT NULL
+) STRICT;
+
+-- Inserts initial metadata row
+INSERT INTO metadata VALUES(0, "");