Per-frame palette calibration + sidebar battery indicator

Advanced configuration (Configuration tab, collapsed <details> section):
a color picker per ink color (black/white/yellow/red/blue/green),
overriding image_pipeline.DEFAULT_PALETTE_RGB for that frame's actual
panel -- different units can vary enough from the documented
approximations to be worth calibrating once you can compare a rendered
photo against the real hardware. Stored as Frame.palette_rgb (NULL =
default, schema migration v4), threaded through render_frame/
render_placeholder/_quantize_and_pack (which now builds the PIL palette
image per call instead of once at import) so both photos and the
unclaimed/unconfigured placeholder screen respect it. "Reset to
defaults" clears back to NULL. Config-save validates exactly 6 #rrggbb
values, rejecting anything else with a 400.

Also: each frame's sidebar entry now shows its last-reported battery
percent (🔋NN%) next to the name, using the frame_dot's existing
recently-seen indicator conventions -- silent when never reported
(mains-only frames, or before the first report), matching how battery
is hidden everywhere else it's not applicable.

Verified against the same live-shaped database as the SMTP work: the
v3->v4 migration, save/reload/reset round trip through the real HTTP
route, an actual rendered image using a custom palette (confirmed via
its packed panel-code bytes), input validation, and the sidebar badge
against real battery data -- plus the standing legacy-device curl suite.

server/app/models.py

@@ -124,6 +124,11 @@ class Frame(Base):
     smart_crop_faces: Mapped[bool] = mapped_column(Boolean, default=True)
     orientation: Mapped[str] = mapped_column(String, default="landscape")
     queue_target_len: Mapped[int] = mapped_column(Integer, default=20)
+    # Advanced configuration: [[r,g,b], ...] x6 (black/white/yellow/red/
+    # blue/green, matching image_pipeline.PANEL_CODES order) overriding
+    # DEFAULT_PALETTE_RGB for this frame's actual panel. NULL = use the
+    # default -- most frames never touch this.
+    palette_rgb: Mapped[list | None] = mapped_column(JSON, nullable=True, default=None)
 
     # -- state --
     current_asset_id: Mapped[str] = mapped_column(String, default="")