fix(realism): synthetic_files path fits MySQL utf8mb4 index cap

The (decky_uuid VARCHAR(64), path VARCHAR(1024)) UNIQUE constraint
generated a 4352-byte composite key under utf8mb4 (4 bytes/char),
busting MySQL's 3072-byte cap and crashing decnet api on init with:

    Specified key was too long; max key length is 3072 bytes

Tighten path to VARCHAR(512) — (64+512)*4 = 2304 bytes, well under
the cap. Real realism + canary placement paths are short
(/home/<persona>/Documents/<file>, ~70 chars); 512 keeps headroom
without the index hassle. Pre-v1, no migration helper.

Adds a regression test pinning the (decky_uuid + path) byte budget so
a future widening fails loudly in CI rather than at MySQL deploy
time.

decnet/web/db/models/realism.py

@@ -58,7 +58,11 @@ class SyntheticFile(SQLModel, table=True):
     )
     uuid: str = Field(default_factory=lambda: str(uuid4()), primary_key=True)
     decky_uuid: str = Field(index=True, max_length=64)
-    path: str = Field(max_length=1024)
+    # Capped at 512 so the (decky_uuid, path) unique index fits MySQL's
+    # 3072-byte utf8mb4 limit: (64+512)*4 = 2304 bytes. Real realism +
+    # canary paths are well under (longest is
+    # ``/home/<persona>/Documents/Q3-Operations-Review.docx``, ~70 chars).
+    path: str = Field(max_length=512)
     persona: str = Field(max_length=128)              # EmailPersona.name
     content_class: str = Field(max_length=32, index=True)  # ContentClass enum value
     created_at: datetime = Field(