Fix bugs, data integrity, and cache busting

- models.py: add UniqueConstraint(user_id, date) to flock_history so
  duplicate flock entries for the same day are rejected at the DB level
- main.py: v2.3 migration applies the new unique constraint to existing
  installs at startup
- login.html: update register form minlength and placeholder from 6 to 10
  characters to match backend; add specific 429 error message so rate-
  limited users see "Too many attempts — please wait a minute" instead of
  a generic failure
- auth.js: update settings modal password input minlength from 6 to 10
- summary.js: fix CSV export truncation — pass limit=10000 so users with
  more than 500 days of data get a complete export; read chart border color
  from --green CSS variable instead of hardcoded hex
- All HTML files: bump JS version params to ?v=4 so browsers discard
  cached copies of files changed across recent sessions (api.js, auth.js,
  dashboard.js, history.js, log.js, flock.js, budget.js, summary.js,
  admin.js)
- .env.example: add password strength guidance for MySQL and admin vars

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/models.py

@@ -30,7 +30,10 @@ class EggCollection(Base):
 
 class FlockHistory(Base):
     __tablename__ = "flock_history"
-    __table_args__ = (Index("ix_flock_history_user_date", "user_id", "date"),)
+    __table_args__ = (
+        UniqueConstraint("user_id", "date", name="uq_flock_user_date"),
+        Index("ix_flock_history_user_date", "user_id", "date"),
+    )
 
     id:            Mapped[int]      = mapped_column(Integer, primary_key=True, autoincrement=True)
     user_id:       Mapped[int]      = mapped_column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)